Thousands have abandoned their homes in northern NSW as the remnants of ex-tropical cyclone Debbie bore down overnight, dumping a staggering amount of rain on the region and causing a major flood emergency in the city of Lismore.

State Emergency Service Deputy Commissioner Mark Morrow said he feared people may have died in the ferocious deluge, which prevented emergency crews from venturing out to rescue those stranded on the roofs of vehicle and houses overnight.

"We couldn't get helicopters in the air, it was too dangerous," Deputy Commissioner Morrow said on Friday morning.

"It was certainly too dangerous in that driving rain to get out there and get to people. So our priority is to try to find whether those people still request our assistance, whether they could get out of the situation that they were in.