The Bureau of Meteorology (BoM) said the Lachlan River and Namoi River in the Central West will experience major flooding over the next 24 hours as a low-pressure system deepens over the state.
It comes after Forbes and Gunnedah have already been hit by damaging floodwaters over the past week.
“Catchments are saturated and the dams are full and conditions are primed for flooding to continue in areas that are already seeing flooding but also to extend to new areas,” meteorologist Jane Golding said.
NSW SES Commissioner Carlene York said there have been more than 6600 requests for assistance over the past 10 days due to the wild weather.
There have also been more than 60 flood rescues in the same time.
In the past 24 hours, the SES has received more than 150 requests for assistance and carried out six flood rescues.
Ms York said Forbes, Gunnedah and Coffs Harbour are the areas hardest hit by flooding.
“As these storms come through and pass there will be very heavy rainfalls in a number of areas, we’re increasing our area of operations,” she said.
Ms York urged people to be prepared for the risk of flash flooding in the coming days.
Weatherzone said some parts of NSW are having the “wettest November in over a century” as a result of La Niña-fuelled rain battering the state.
Orange has recorded more than 243mm of rain this month, the most since records began in 1870.
Condobolin recorded more than 131mm, the most since 1881.
“With another 50-100mm of rain expected across a broad area of NSW later this week, more places will likely set new November records by the end of the month,” Ben Domensino from Weatherzone said.
Brisbane has also broken records, reporting its heaviest single-day rainfall in November for 40 years.
More than 120mm fell to 9am yesterday.
“Not since 1981 has more rain been recorded on one day in November,” Anthony Sharwood from Weatherzone said.
Widespread showers and thunderstorms, with the risk of heavy rainfall, are expected to hit most of Queensland tomorrow and Friday.
According to meteorologists, atmospheric moisture left behind in the system that crossed northern NSW and southern Queensland at the weekend, combined with winds circulating, has brought moisture over eastern parts of the country.
Other reasons for the wet weather include humid tropical air that is streaming across the country from the Indian Ocean, where Tropical Cyclone Paddy was named yesterday.
There is also another low-pressure system spinning over the Great Australian Bight.
BoM meteorologist Jackson Brown described La Niña as the movement of warm waters across the Pacific Basin.
“We have warm waters sitting off the east Australian coast and with those warm waters, you see additional energy available to drive a wetter system,” he said.
“It makes heavy rainfall a lot more likely through vast stretches of eastern Australia.”
People in NSW and Queensland have been advised to stay alert as many areas were at risk of severe thunderstorms and flooding.
Showers are forecast in Brisbane today with a top of 26 degrees, with a shower or two also expected in Sydney and a maximum temperature of 26 degrees.
Rain is also expected in Melbourne, Adelaide, Hobart and Canberra today, while in Perth and Darwin it will be warm and sunny.