The Bureau of Meteorology has numerous flood warnings and has also forecast possible severe thunderstorms, hazardous surf, damaging wind and hail.

"What we are faced with is a very serious situation on broad areas of the NSW east coast," the BOM's Dean Narramore said on Monday.

The warnings extend to "every community and town in between and extending well inland towards the Blue Mountains, Central Tablelands, Southern Highlands and possibly Canberra", he said.

The SES ordered residents in Camden, in southwest Sydney, to evacuate on Monday night due to the danger of flooding. Parts of Emu Plains, Penrith and Mulgoa in far west Sydney have also been advised to prepare for evacuation.

For Will Potter from North Richmond, the record-breaking floods are a grim reminder of previous natural disasters and forthcoming ones.

"There are families that have had to rebuild their lives from the floods for three years in a row now. I'm just down the road from where the fires were in 2019," the 20-year-old told environmental group Getup.

"It just feels like we're on our own."

Fire and Rescue crews were dispatched on Monday to the scene of a major landslide, caused by intense rainfall, at Katoomba in the Blue Mountains.

FRNSW said the rail line was also in danger as Greater Sydney braces for more rain.

State Emergency Commissioner Carlene York warned the Hawkesbury River was of prime concern for Tuesday.

"We're looking at flooding similar to last week on some parts the river. In other areas it could be worse than what we saw last weekend - similar to what we saw in March last year," she said.

"If it is not safe to go home please do not go home."

Soils are saturated and rivers swollen, so severe thunderstorms and persistent showers are more likely than usual to lead to landslides, flash and riverine flooding.

The heavy rain is forecast to increase across the Hunter on Monday and move towards Sydney, Hawkesbury Nepean and spread to the Illawarra and Shoalhaven.

Heavy rain will ramp up in the early hours of Tuesday and continue most of the day, Mr Narramore said.

A road weather alert has been issued for metro Sydney through to the Central Coast and Illawarra, with a warning heavy rain and strong winds could down trees and powerlines, cut roads and cause land slips.

The warnings come as Major General David Thomae defended the performance of the Australian Defence Forces' rescue efforts, which have come under fire from flood-devastated communities.

"I am very sorry for all of those people who have felt that they haven't been supported. I empathise completely with their plight," he told reporters in Sydney.

"The scale of devastation is extraordinary. It is something that is very sobering for all of us to look at."

He said more than 100 people had been rescued off rooftops and that 637 ADF personnel are in northern NSW. A further 656 to be dispatched by Tuesday to help with everything from food drops to cleaning up.

Premier Dominic Perrottet apologised to thousands of people in the Northern Rivers whose homes and businesses were destroyed when towns like Lismore, Ballina and Mullumbimby were inundated last week.

Many had to be rescued by fellow citizens and have since struggled to access basics such as food, water, power, fuel, phones, ATMs, the internet and medical supplies.

"I'm very sorry ... but we're doing everything that we can," Mr Perrottet told the Nine Network from Lismore.

"Some of those areas will look back at this and say it could have been done better but ... we're doing absolutely everything we can to provide care and support for those who need it to get those essential supplies in," he said.

Around 2000 of the 3500 homes assessed so far in the Northern Rivers are no longer habitable, a number expected to rise, increasing the need for short-term and medium-to-long-term accommodation.

Floods have claimed six lives in NSW, including four in Lismore.

Australian Associated Press