Footage has emerged showing the terrifying moments a woman and her niece managed to make a last second escape from their sinking 4WD after being swept off the road by floodwaters near Lismore.

The 67-year-old woman told Nine News it took all of 15 seconds for her and her niece to realise they were in trouble after becoming trapped in the floodwaters at Boatharbour Road as she drove from Byron Bay to Lismore.

Flooding on Boatharbour Road

UPDATE: Bangalow Road now open to traffic after earlier flood closure

“Oh my God, holy crap,” a woman filming in a trailing vehicle can be heard saying in the footage.

“What are we going to do?” Her father can be heard asking as the car stops at the edge of the floodwaters.

“Oh my God, their car is literally rolling off. They need to get off. Their car is sliding, they need to get out,” she says as her father wades in to help.

“It’s gone! It’s literally gone,” the woman continues in commentary as the pair begin walking away from their submerged 4WD.

The Bureau of Meteorology’s acting state manager for NSW, Jane Golding, said the rain was set to intensify in the coming hours.

“This is a very dangerous system,” Ms Golding said.

“We’ve been given a bit of breathing space but it’s just starting to increase now, this is the most intense development of the system.”

She warned of significant overland flooding, flash flooding and landslips, adding that the recent bushfires in some areas have made landslips more likely.

“This rain is starting to become more widespread and the more intense showers within it are starting to become more frequent,” Ms Golding said.

She added some places could receive more than 200mm of rain, which would be backed up by another “very wet day tomorrow with higher rainfall totals”.

“We are seeing the signs that an east coast low will develop,” Golding said, which would bring gale force winds to some parts of the coast.

A King tide is expected on Monday morning which will increase the risk of flooding.

A coastal trough resulting in widespread heavy rainfall across the State continues today. An east coast low continues to develop as part of this system currently off the middle of the NSW coast, forecast to track southwards later today and into tomorrow.

Northern Rivers NSW SES said late this afternoon the area's catchments are becoming saturated and further rain will result in quick rises of affected rivers, however steady rain has been absent, apart from isolated falls, some associated with thunderstorms.

The thunderstorms have caused some local road closures due to pockets of flash flooding in the Richmond and Clarence Valleys. There is potential for further rises in streams as a result of the thunderstorm activity. River levels across the Northern Zone are continuing to slowly fall, Northern Rivers SES said.

"Rainfall forecast for the area is abating at this point, we are postured to continue operations at this level and poised to ramp up with a close watch on the forecasts," Northern Rivers NSW SES said.