East coast residents have been warned to stay vigilant as a severe weather system delivers a likely deluge and possible flooding.

The NSW north coast was expected to bear the brunt of wild weather as it moved across the border from Queensland on Monday, the Bureau of Meteorology said.

In the NSW Northern Rivers region, Ballina had received about 94 millimetres of rain between 9am and mid-afternoon, while Yamba to the south had received nearly 80 millimetres.

Rising local rivers and flash flooding were likely within the areas that received the heaviest falls, the bureau said.

Catchments within the flood-watch areas had been dry but were beginning to become wet as widespread rain and isolated thunderstorms continued in parts of coastal Queensland and north-east NSW, meteorologist Dean Narramore said.

Heavy rainfall is likely to affect regions in an area stretching to as far north as Mackay in Queensland.

“We have an upper trough combined with a really moist onshore flow and that’s causing this trough system to deepen and cause widespread rain and storm activity from the central [Queensland] coast all the way down to north-eastern NSW,” Narramore said.

“It’s going to continue today, tonight and into tomorrow as this system very slowly moves south.”

The weather system was forecast to move off the coast by late Wednesday or Thursday.

Forecaster Weatherzone said there were flood watches between central Queensland and north-east NSW as the unseasonably heavy rain continued.

“This drenching kicked off on the weekend and will continue during the first half of this week,” it said on Monday.

Some areas have already had their heaviest August rain in decades, with 157 millimetres at Evans Head and Repentance in NSW and 138 millimetres at Upper Major Creek in Queensland.

“In Queensland, Ayr’s 67.8 millimetres during the 24 hours to 9am on Monday was its heaviest August daily rainfall in 53 years. Townsville Airport’s 67.2 millimetres was also a 26-year high for an August day,” Weatherzone said.

It said some areas of central Queensland and north-east NSW could get as much as 200 millimetres more rain in coming days.

“A few places might even pick up 200-300 millimetres, particularly where rainfall is enhanced by local topography or thunderstorms,” it said.

“Rain will ease over central Queensland from Wednesday and most other areas of eastern Australia by Thursday as the upper-level lows weaken and move offshore.”

The NSW State Emergency Service has urged residents in Lismore, Byron Bay, Ballina, Evans Head, Yamba and Maclean to monitor conditions.

Rainfall totals between 40-80 millimetres were likely in those areas on Monday, with isolated totals possible of up to 150 millimetres.

“Currently, rain is occurring across the Northern Rivers and is expected to persist until this afternoon,” the SES said.

-with AAP