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The NSW premier is urging residents to follow safety advice and avoid driving through flooding, after State Emergency Service volunteers had to rescue more than 30 people from dangerous waters overnight.

The SES staged 31 flood rescues between 12pm Friday and Saturday afternoon.

"That is way too high," Dominic Perrottet told reporters.

"Those rescues take place in the main because people aren't following the instructions, they're driving through floodwaters. Don't do that. Follow the advice, that's the best way to stay safe."

More than 500 SES volunteers are on the ground, with 745 requests for help across NSW since lunchtime on Friday.

Residents of low lying parts of the Whittingham, Scotts Flat, Glenridding, Dunolly and Combo areas along the Hunter River are being told to prepare to evacuate.

The SES says they may need to evacuate if flood waters rise further, and they are at risk of flash flooding.

In the central west, Eugowra residents are being told to prepare to evacuate as water levels rise along the Mandagery Creek.

On Saturday morning, SES Assistant Commissioner Dean Storey described a hectic 24 hours throughout the state but particularly in the Upper Hunter.

"That continues to be where our focus is today," he told ABC TV.

"Scone and Muswellbrook were affected by floodwaters over the last 24 hours and will continue to be to an extent today, with the focus shifting downstream."

Major flooding is on the cards in Singleton for Sunday morning, and Maitland is also on alert.

Rain eased on Saturday afternoon, with not much forecast for Sunday.

However, the statewide situation remains volatile, with numerous watches and warnings active following a month of heavy rainfall.

Areas of major concern include along the Namoi River at Gunnedah, in the state's northeast, and the Castlereagh River around Molong in the central west.

"This is quite a large statewide flood risk at the moment," Mr Storey said.

"We are urging all communities and those flood prone areas to be aware of the risk and monitor the local conditions and follow the advice."

Sydney's Warragamba Dam began spilling shortly after 9pm on Friday.

WaterNSW says the outflow volume could peak at a rate of about 60-80 gigalitres per day, just a fraction of levels in March that peaked at 500 gigalitres per day.

"On current projections the spill could continue for up to a week," WaterNSW said in a statement on Saturday, adding "downstream impacts are likely".

A flood watch is in place for the Hawesbury Nepean valley in western Sydney.

The Bureau of Meteorology said some areas of the state were hit with more than 100mm of rain on Friday.

There were major flood warnings for the Mooki River at Gunnedah, the Castlereagh at Coonamble, the Lachlan at Jemalong and the Namoi at Gunnedah, Narrabri and Wee Waa, where it's feared residents could be cut off for more than a week.

Police said a Landcruiser was swept from a small causeway and carried 200 metres downstream by rising waters at Burrell Creek, west of Taree, on Friday afternoon.

Two men waded in to rescue the 65-year-old trapped driver but also became stranded. The trio were eventually hauled to safety by five police officers who fashioned a lasso from a length of rope.

Hunter Valley police also dragged a 52-year-old man from a spillway at Muswellbrook after his Toyota Hilux was swept 400m away by the current on Friday evening.

An RFS helicopter rescued a man trapped on the roof of his car in floodwaters at Caroona on the northeastern Liverpool Plains earlier on Friday.

Mr Storey said it was a good time to stay off the roads.

The Oxley Highway is closed in both directions between Carroll and Gunnedah and the Kamilaroi Highway between Curlewis to Breeza.

At Muswellbrook, the New England Highway is closed in both directions.

In the state's north, severe thunderstorms are forecast to develop around Glen Innes and Inverell on Saturday evening, which could lead to flash flooding.

© AAP 2021

Australia has shut its borders to nine southern African countries and suspended all flights from that region amid growing concerns about the new Omicron variant of COVID-19.

Federal Health Minister Greg Hunt says Australia is in a vastly different position to other countries due to high vaccination rates, but precautionary measures are needed against the new strain.

He says all flights have been immediately suspended for two weeks from South Africa, Namibia, Zimbabwe, Botswana, Lesotho, Eswatini, the Seychelles, Malawi, and Mozambique.

Any Australian citizens or dependents who have been been in those countries in the past 14 days must go into mandatory quarantine, and non-citizens who have been southern Africa will be banned from entering Australia.

Mr Hunt says anyone who has already arrived in Australia from any of those nine countries must go into immediate self-isolation.

"There are no known cases of the Omicron variant in Australia," the minister told reporters on Saturday.

"We've taken precautious action in the past, we've taken early action in the past, we are doing that again.

"The difference is that we now have strong vaccines, we have one of the highest levels of coverage in the world, we have one of the most recently vaccinated populations in the world, and we have strong public health and social measures."

The number of people who have arrived from southern African since November 1 is thought to be under 100.

The latest variant, given the name Omicron by the World Health Organisation on Saturday morning, first emerged in Botswana and has been detected in South Africa, Hong Kong, Israel and Belgium.

It has double the number of mutations as the Delta variant that sparked a third wave of outbreaks and lockdowns in Australia this year.

Chief Medical Officer Paul Kelly said Omicron was spreading quickly in southern Africa, but it wasn't clear yet that it has more severe symptoms than other strains or if it can evade immunity from vaccines.

"They're crucial points, it's the reason why we're taking this precautionary approach, which is proportionate to that risk," he said.

Professor Kelly also said it was premature to be talking about whether domestic lockdowns or further restrictions would be needed to stop the spread of Omicron within Australia.

So little was known about the variant, including which public health measures could be effective in containing it, he said.

"In terms of ruling in or out what else we might do, as the minister has clearly said, we will do what we need to do at this stage," Prof Kelly said.

"To move toward speculation about where we might end up with in Australia, even if it came here, and we don't have it here yet, that is premature."

Mr Hunt said the new travel restrictions were strong and swift, but precautionary, and they can easily be rolled back or ratcheted up if needed.

The government could close borders and suspend flights to additional countries, he said, if the variant spreads.

"If more actions are required, we will not hesitate," the minister said.

"Today we have taken those actions, today I'm saying, if the medical evidence shows that further actions are required, we will not hesitate to take them and that may involve strengthening or expanding the restrictions."

About 86 per cent of Australians aged 16 and older are double-dosed, which means between 72 and 73 per cent of the entire population. Just 1.5 per cent of the country have received a booster shot.

The federal government is sending letters to every household in the country urging people to get their booster shot six months after becoming double-dosed.

© AAP 2021

The NSW premier is urging residents to follow safety advice and avoid driving through flooding, after State Emergency Service volunteers had to rescue more than 30 people from dangerous waters overnight.

"We've seen over 100 flood rescues just in relation to the more recent floods (and) 34 overnight," said Premier Dominic Perrottet on Saturday morning.

"Well that is way too high.

"Those rescues take place in the main because people aren't following the instructions, they're driving through floodwaters. Don't do that. Follow the advice, that's the best way to stay safe.'

More than 500 SES volunteers are on the ground, with almost 600 requests for help across NSW since lunchtime on Friday.

SES Assistant Commissioner Dean Storey says it has been a hectic 24 hours throughout the state but particularly in the Upper Hunter.

"That continues to be where our focus is today," he told ABC TV on Saturday morning.

"Scone and Muswellbrook were affected by floodwaters over the last 24 hours and will continue to be to an extent today, with the focus shifting downstream."

Flooding is on the cards in Singleton on Saturday evening and then at Maitland into Sunday, Mr Storey said.

However the statewide situation remains volatile, with numerous watches and warnings active following a month of heavy rainfall.

Areas of major concern include along the Namoi River at Gunnedah, in the state's northeast, and the Castlereagh River around Molong in the central west.

"This is quite a large statewide flood risk at the moment," Mr Storey said.

"We are urging all communities and those flood prone areas to be aware of the risk and monitor the local conditions and follow the advice."

Sydney's Warragamba Dam began spilling shortly after 9pm on Friday.

WaterNSW says the outflow volume could peak at a rate of about 60-80 gigalitres per day, just a fraction of levels in March that peaked at 500 gigalitres per day.

"On current projections the spill could continue for up to a week," WaterNSW said in a statement on Saturday, adding "downstream impacts are likely".

The Bureau of Meteorology said some areas of the state had been hit with more than 100mm of rain on Friday. The downpour was likely to ease over the weekend but the risk of flooding persists.

There were major flood warnings for the Peel River at Tamworth and the Namoi at Gunnedah, Boggabri, Narrabri, Bugilbone, Goangra and Wee Waa, where it's feared residents could be cut off for more than a week.

Police say a Landcruiser was swept from a small causeway and carried 200m downstream by rising waters at Burrell Creek, west of Taree on Friday afternoon.

Two men waded in to rescue the 65-year-old trapped driver but also became stranded. The trio were eventually hauled to safety by five police officers who fashioned a lasso from a length of rope.

Hunter Valley police also dragged a 52-year-old man from a spillway at Muswellbrook after his Toyota Hilux was swept 400m away by the current on Friday evening.

An RFS helicopter was called in to rescue a man trapped on the roof of his car in floodwaters at Caroona on the northeastern Liverpool Plains earlier on Friday.

Mr Storey said it was a good time to stay off the roads.

The Oxley Highway is closed in both directions between Carroll and Gunnedah and the Kamilaroi Highway between Curlewis to Breeza.

At Muswellbrook, the New England Highway is closed in both directions.

© AAP 2021