More heavy rain is forecast across large swathes of NSW in coming days, threatening to prolong the flood crisis that has inundated towns, isolated communities and led to the death of a young boy.

The Bureau of Meteorology warned on Monday that a trough moving across NSW could prolong the flooding, with widespread showers and thunderstorms predicted across much of the state, especially on Tuesday.

The rain is likely to stretch along a massive part of Australia’s east, bringing soakings as far south as Victoria – described by the bureau as an “evolving situation”.

“Severe thunderstorms are forecast for coastal areas from the Queensland/NSW border to Port Macquarie on Friday, with the chance of thunderstorms extending further south to the Blue Mountains on Saturday,” the BOM said in a statement on Monday.

“This will bring the increased risk of flash flooding, hazardous driving conditions, and gusty winds may bring down trees and powerlines.”

In NSW, communities were braced for more destructive rain as the BOM issued minor to major flood warnings in already soaked catchments. Some areas have already had more than twice their average September rainfall.

On Monday afternoon, the weather forecaster warned the low-pressure system may also move over the southern coast on Wednesday.

“Heavy rainfall and strong winds are possible somewhere along the southern half of the coast, but there’s some uncertainty for the exact location,” it said on Twitter.

In NSW’s north, the towns of Warren and Wee Waa remain cut off and expected to remain isolated for days as the flood emergency continues.

Further east, the town of Gunnedah continued to be affected by flooding but the water was slowly receding.

The Namoi River, which runs through Gunnedah, peaked at 8.24 metres at the weekend. SES workers were assessing the damage on Monday.

The crisis has claimed the life of a five-year-old boy.

He was found dead on Saturday afternoon after the vehicle he was in got swept away at Tullamore, near Parkes. His parents reportedly freed two other children from their vehicle, but could not save the boy.

Emergency services have received hundreds of calls for help since the flooding hit the region last week.

SES Assistant Commissioner Nicole Hogan said NSW’s central-west and north-west regions remained at risk of further flooding for the next six to eight weeks.

With more rain forecast this week, Ms Hogan said it was crucial that residents and those travelling in the region took care.

“We are asking people, particularly with the school holidays, to prepare their route, [and] know where they’re going ahead of time … and if they are camping on the side of a river, to really be aware of their surroundings,” she told ABC television on Monday.

“Communities just really need to understand their flood risk, know what the conditions are like, be aware of their surroundings.”

Tweet from @EWNAlerts

On the north coast at Lismore, which was devastated by floods earlier this year, NSW Labor leader Chris Minns said the key message from the community was that promised support had to hit the ground.

Lismore was submerged in record-breaking floods in February and March. It was also threatened again last week.

“The funds need to come, if they don’t come you’ll start to see businesses shut their doors and they won’t return,” Mr Minns said.

Elsewhere, there were flood warnings for several Victorian rivers on Monday. The bureau warned that river rises and minor flooding was possible as heavy rain moved into the southern state.

-with AAP