Rising floodwaters have overflowed Lismore’s levee, with the town’s central business district facing its second devastating inundation in less than a month.

Heavy rain earlier forced a new evacuation order for the northern rivers city, with CBD residents and those in surrounding Lismore Basin, East Lismore and Girards Hill urged to leave immediately.

“Everything’s falling apart in Lismore at the moment,” Lismore Mayor Steve Krieg told the Nine Network on Wednesday morning.

Later Cr Krieg confirmed the river had begun to tip over the top of the levee.

“Everyone’s exhausted,” he said.

“Evacuation orders started at four o’clock yesterday afternoon and then got cancelled before the phones started ringing again.

“[People are] running on no sleep at the moment.

“It’s hard to put into words. You’re feeling angry as well because the messaging hasn’t been what it should have been.”

Elsewhere, Byron Bay’s main street was also underwater on Wednesday morning. The tourist mecca endured torrential downpours overnight.

Back in Lismore, Cr Krieg said frustration grew after an evacuation order was cancelled half an hour after being issued on Tuesday.

It was then re-issued late Tuesday night, with the Wilson expected to burst its levee on Wednesday, peaking over major flood levels of 10.6 metres, up to 11 metres and higher, BoM said.

The worst might still be to come, with even higher readings at Lismore possible on Wednesday afternoon.

Like many Lismore locals, Cr Krieg has been sleeping at a friend’s home for the past month. He  said he would bunk with others for months to come.

He said the silver lining about this latest round of flooding was that most people lost the bulk of their valuables in the first inundation.

“The hardest thing for these people is they have spent weeks and weeks cleaning and getting rid of all the flood mud, sanitising and addressing the mould issue,” Cr Krieg said.

“Now you’ve got to virtually start all of that process from the start again.”

Byron, the owner of a petrol station in Murwillumbah said his business was flooded in the last catastrophic event in the Northern Rivers earlier this month.

“It came up to like two metres [high] in the shop and we’ve lost everything,” he said.

The most recent floods had broken the business’ petrol pumps, ovens and fridges, which had all been replaced since then.

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Flash flooding in Lismore’s CBD has again inundated roads and an evacuation centre has been set up at Southern Cross University.

Another bout of flooding could seriously affect those in the middle of a long clean-up from the recent flooding catastrophe, University of Sydney mental health professor James Bennett-Levy said on Tuesday.

“There is extreme distress because what it does is re-trigger and re-traumatise people who have already been severely traumatised.”

Dr Bennett-Levy, who directs the Centre for Rural Health in Lismore, said “very high levels” of post-traumatic stress disorder were expected.

“It is not just people directly affected, there is collective trauma because just about everyone in the community knows … multiple people … whose houses have been inundated,” he said.

In a study conducted after the 2017 floods, Dr Bennett-Levy along with other researchers found 50 per cent of people displaced for more than six months in the same region had PTSD.

-with AAP