Premier Gladys Berejiklian has warned that "complacency is our biggest challenge".

"While rivers may be reaching their peak, the flows and the current are very dangerous," Ms Berejiklian told parliament on Thursday.

She urged people in affected areas to be "extra cautious".

"Benign" conditions will hang around the east coast for at least the next couple of days, the Bureau of Meteorology says. However, the floodwaters have not yet receded.

There were still 17 evacuation orders in place on Thursday afternoon, with about 21,000 people kept from their homes. Between 3000 and 4000 people returned to their homes over the past 24 hours, the premier said.

"We know that many people are confronted by very catastrophic conditions and their lives will never be the same," she said.

The sheer volume of rain that fell across the state will take quite some time to work its way through the river systems, hydrologist Victoria Dodds says.

"We've seen exceptionally high flood levels, high-velocity flows, lots of dangerous debris in these floodwaters," she told reporters on Thursday.

Ms Dodds said flood warnings would likely remain in force across the state, particularly in inland areas, for the days and weeks ahead.

Ms Berejiklian pleaded with people to stay out of floodwaters after two drivers died trapped in their cars.

A man died in Glenorie in northwest Sydney on Wednesday and the body of another man was found submerged in a ute in Queensland's southeast.

There have been 11,000 calls for help to the NSW State Emergency Service, and 950 flood rescues.

Fire and Rescue NSW have also rescued three people who were swallowed by a sinkhole near the Mehi River in Moree, where major flooding is occurring.

There is still significant flooding along a number of rivers, but the focus has turned to the northern rivers region, particularly Grafton, Maclean and Ulmarra.

Those in low-lying areas of Ulmarra, Bushgrove and Cowper were ordered to leave on Wednesday afternoon.

Major flooding is also occurring along the Hawkesbury River and authorities say it's likely to continue in North Richmond and Windsor until the end of the week.

Moree, the Upper Hunter around Singleton and parts of the Central Coast are still of concern.

More than a dozen evacuation orders remain, and about 60,000 people are on standby to evacuate, with major flood warnings still in place for the Macintyre, Gwydir, Clarence, Hawkesbury, Nepean and Colo rivers.

Ninety schools remain closed across the state. At least 50 have permanent damage, Ms Berejiklian says.

Federal Energy Minister Angus Taylor said while tens of thousands of customers have had their electricity restored, about 3600 customers were still without power.

Access to flood areas remains difficult, with some cables still submerged.

With no major rain forecast for at least a week, the huge task of cleaning up can begin.

Defence and emergency service personnel are flying in essential supplies to isolated communities, particularly North Richmond.

Helicopters delivered 54 pallets of food to the Richmond community on Wednesday.

ADF members have already been embedded in emergency operations centres in areas of concern, and are part of teams assisting with damage assessments.

The defence force's role will be stepped up to a force of about 700, with troops brought in from northern NSW and Newcastle.

Australian Associated Press