New South Wales residents have been warned to brace for flooding by the Bureau of Meteorology.
BoM National Flood Operations Manager Justin Robinson said he is "really concerned" about north western NSW during his flood update on Tuesday afternoon.
"The area we're most concerned about are those areas in the north western inland parts of the state, which have been impacted by flooding in recent weeks," Mr Robinson said.
"That means these communities have already seen major flooding and there's more rainfall on the way over the next two days and we're really concerned that we might see some renewed flooding and renewed river level rises after that rainfall."
A flood watch was issued for the NSW Northern Rivers and Mid North Coast on Tuesday morning while the BoM has also flagged renewed river level rise in the North West Slopes.
North Coast towns such as Coffs Harbour, Lismore and Ballina have been warned to brace for two-day rainfall totals of 80 to 120 millimetres.
Meanwhile, northern inland towns such as Moree, Inverell and Narrabri are forecast to receive 40 to 80 millimetres over the same two-day period.
Mr Robins said the BoM is "most concerned about" flooding in the Upper MacIntyre River and Gwydir River.
He said areas that have already suffered "very significant flooding" such as Gunnedah and Wee Waa were also of concern.
The Flood Operations Manager said flooding could continue into the next week "depending on how much rain we get over the next couple of days".
"It does take a while for these floodwaters to move down through those river systems. So I expect that to continue well into next week. But we're really hoping to give a bit of respite from some of the rainfall," Mr Robinson said.
"But every day we seem to be getting a little bit more rainfall across those flood affected areas."
Mr Robinson advised people in affected areas to "stay out of the flood waters".
"It's just really important. But also important is actually to keep across the bureau's warnings and the advice from the SES. So the bureau is monitoring the flood situation at all times and when we see the situation change, we'll update those warnings as soon as possible," he said.
"And normally once we start seeing the rain, we will start issuing those warnings every three to six hours with updates for the community."
Mr Robinson said "people should definitely be aware of the dangers of flash flooding" as parts of the state receive more than 100mm of rain.
"If you get some very heavy rainfall and very wet catchments, you will see some flash flooding and flash flooding can be particularly dangerous, especially if you just happen to be in the wrong place at the wrong time where you get some really heavy, isolated rainfall," he said.
There is currently a major flood warning for the lower Macintyre River and flooding warning for the Weir River in both NSW and Queensland.
There are also minor to major flood warnings for the Lachlan, Macquarie and Namoi rivers in NSW.
A number of minor, moderate or final flood warnings are also in place for Victoria, the ACT, NSW and Queensland, which can be viewed on the BoM website.