More than 4,000 people have been cut off by rampant floodwaters in northern NSW, with swollen rivers still far from reaching their peaks.

Premier Kristina Keneally will on Wednesday visit some of the flood-affected communities, with the SES saying some 4,500 people are now isolated.

More than 150 millimetres of rain has been dumped in the past 24 hours over the state's northeast.

"The heavy rains are having an impact on northern NSW communities, resulting in flash flooding and isolations," Ms Keneally said in a statement issued on Tuesday.

"Tomorrow I expect to travel to some of the flood-affected communities and directly see the damage and the flood response efforts myself."