The Bureau of Meteorology (BoM) said the Lachlan River and Namoi Rivers in the central west will experience major flooding in the next 24 hours as a low pressure system deepens over the state.
It comes after Forbes and Gunnedah already suffered harmful floods last week.
“The river basins are saturated and the dams are full and the conditions are prepared for flooding to continue in areas where there is already flooding but also to expand to new areas,” said meteorologist Jane Golding.
NSW SES Commissioner Carlene York said there had been more than 6,600 requests for help in the past 10 days due to the wild weather.
At the same time there were more than 60 flood rescues.
In the past 24 hours, the SES has received more than 150 requests for help and carried out six flood rescues.
Ms. York said Forbes, Gunnedah and Coffs Harbor were the areas hardest hit by flooding.
“When these storms come through and pass, there will be very heavy rains in a number of areas, we will expand our operational area,” she said.
Ms. York urged people to prepare for the danger of flash floods in the coming days.
Weatherzone said some parts of NSW are having “the wettest November in over a century” as a result of the La Niña-fueled rains that hit the state.
Orange recorded more than 243mm of rain this month, most since records began in 1870.
Condobolin recorded more than 131mm, most since 1881.
“With another 50-100mm of rain expected across a large area of NSW later this week, more locations are likely to break new November records by the end of the month,” said Weatherzone’s Ben Domensino.
Brisbane also broke records, reporting the heaviest one-day rainfall in 40 years in November.
More than 120mm fell at 9 a.m. yesterday.
“We haven’t seen rain in a November day since 1981,” said Weatherzone’s Anthony Sharwood.
Widespread showers and thunderstorms, with the risk of heavy rainfall, are expected to hit most of Queensland tomorrow and Friday.
According to meteorologists, the moisture in the system that crossed north New South Wales and south Queensland over the weekend, combined with the circulating winds, brought moisture over the eastern parts of the country.
Other reasons for the wet weather are humid tropical air flowing over the country from the Indian Ocean, where Tropical Cyclone Paddy was called yesterday.
There is also another low pressure system that spins over the Great Australian Bight.
BoM meteorologist Jackson Brown described La Niña as the movement of warm water through the Pacific Basin.
“We have warm water off the East Australian coast and that warm water gives us extra energy to power a wetter system,” he said.
“It makes heavy rains much more likely in much of Eastern Australia.”
People in NSW and Queensland were advised to remain vigilant as severe thunderstorms and flooding threatened many areas.
In Brisbane today showers are forecast with a maximum temperature of 26 degrees, with one or two showers also in Sydney and a maximum temperature of 26 degrees.
Rain is also expected in Melbourne, Adelaide, Hobart and Canberra today, while Perth and Darwin will be warm and sunny.