The Bureau of Meteorology has numerous flood warnings along the coast 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.
State Emergency Commissioner Carlene York warned the Hawkesbury River was of prime concern.
"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.
"Stay where it is safe and make decisions that are safe for you and your families."
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.
"We are likely to see major flooding on numerous rivers, much of the flooded catchments that does include the Hawkesbury and Nepean."
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 communities in the Northern Rivers continue to mop up from last week's unprecedented flooding.
Ms York said the SES was delivering essential supplies to flood isolated communities and 5000 members of the ADF would arrive in the coming days to help with the clean-up.
"We have resources from the ADF that they will be 2000 on the ground by the end of the day into tomorrow."
Premier Dominic Perrottet has 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.
The SES received 1290 calls for help in the last 24 hours and carried out 25 rescues in the same period.
Job protection has been announced for flood emergency volunteers, to protect people from being sacked or demoted as they help with the clean-up.
The cost of insurance claims in the state has risen to $312 million, the Insurance Council of Australia said, after the total estimated cost of claims in NSW and Queensland rose to $1.3 billion.
Some 86,703 claims have been lodged, 28 per cent from NSW.
Telstra said it had restored about 80 per cent of mobile coverage in northern NSW and 75 per cent of landline connections, however continued flood and road damage continued to hamper their technicians.
