Extra flood rescue crews are being moved to NSW's south and northeast as heavy rain and widespread storms continue to drench the state.
Heavy rainfall leading to flash flooding is possible in Albury, Yass and other parts of southern inland NSW from late Wednesday morning, the Bureau of Meteorology has warned.
Falls of 50 to 80mm within six hours are possible while isolated areas may experience intense downpours of more than 100mm, leading to life-threatening flash flooding.
The State Emergency Service urged all people in the areas, including campers to stay out of floodwater and monitor conditions.
"There is a chance of flash flooding, which can happen quickly and without warning," Assistant Commissioner Nicole Hogan said.
"We know there are a lot of people travelling at this time of year and I urge people to be mindful of any risks associated with camping in floodplains or near rivers."
The storm activity is being caused by a very humid airmass drawn from the tropics moving over southern NSW.
"This airmass will lift ahead of an approaching cold front from Wednesday morning, resulting in areas of rain with embedded heavy shower and thunderstorm activity," the BoM said.
It comes as a handful of towns in the Clarence Valley are isolated by flood water after widespread deluges.
Residents near the village of Nymboida, close to Grafton, were warned on Tuesday evening the flooding Nymboida River would likely isolate properties.
Properties along the Orara River also face isolation.
Among the highest falls in the northeast was Dorrigo, west of Coffs Harbour, which received 380mm between Monday morning and Tuesday afternoon while Bellingen copped 287mm and Byron Bay 162mm.
The NSW State Emergency Service responded to 75 incidents over the period in the state's northeast.
Four rescues included three missions to save people in vehicles trapped in floodwaters. All of those were safe.
Statewide, more than 460 incidents were called in to the SES, including hundreds in the Sydney-Newcastle-Wollongong area.
Numerous warnings were issued in the north east, including watch and act directions for rivers in the Coffs Harbour area.
A number of waterways north of Port Macquarie and in the state's northwest were at advice level, with further rainfall potentially worsening conditions.
