Two vehicles, including a Toyota Hilux, became trapped in floodwaters at Tullamore, northwest of Parkes, on Friday night.

Emergency services used a boat to rescue four people from the Hilux who were found clinging to trees, including a 37-year-old man, a 28-year-old woman, and a young boy and girl.

However, the five-year-old boy was still trapped in the car when it was swept away.

NSW Police officers started searching on Saturday for the Hilux and the boy's body was found by police divers at about 3.20 pm in the submerged car.

His family was taken to Dubbo Base Hospital for observation. The occupants of the second vehicle weren't injured.

The Hilux has since been retrieved and police are investigating ahead of a report being prepared for the coroner.

Chief Inspector David Maher said the family was distraught, in comments reported by the ABC.

He said the parents had been able to remove the car restraints from the younger children and pull them out, but weren't able to free the older boy.

Tullamore has been hit by heavy rainfall in recent days.

A low-pressure system that brought torrential rain and flooded dozens of river systems in inland NSW and the state's north in the latter part of the week was moving offshore on Saturday.

Weather easing off in the state's northeast was welcome news for those communities, however there was an ongoing flood risk for areas of western NSW, and west of the Great Dividing Range, the State Emergency Service said.

Major flooding was also happening on Saturday in the Bogan, Macquarie, Gwydir, and Lachlan catchments, as well as the Narran and Macintyre Rivers.

Bureau of Meteorology forecasters were turning their attention to Tuesday and Wednesday, when more rain was anticipated. Northern and inland areas were expected to be spared the worst of it.

"People in the catchments a little bit further south (and in) central areas like the Bogan and lower Lachlan catchments ... could get some considerable rain," senior meteorologist Jake Phillips said.

In the town of Gunnedah, home to 9000 people, the Namoi River peaked at the major flood level of 8.24 metres on Saturday morning.

The water was progressively going down, SES spokesman Greg Nash said.

"We ask people that are currently downstream of places like Gunnedah on the Namoi River ... to keep advised of information," he said.

"It's anticipated that water will move downstream slowly and may isolate some areas."

At Wee Waa, just 120km northwest, major levels of flooding would persist for at least a week, Mr Nash said.

The cotton town is protected by an 8km levee but Narrabri Shire mayor Ron Campbell told AAP the rainfall had destroyed local roads.

"If we get substantial rain across the summer, we could have a record flood for sure - probably something not seen since the 1970s," Mr Campbell said.

The SES has additional resources in Wee Waa and was starting to see resupply requests, including for the local pharmacy, Mr Nash said.

The town of Warren, about 250km southwest of Wee Waa, will also experience major flooding for at least a week, he said.

A severe thunderstorm warning was cancelled on Saturday afternoon for the mid north coast, while flooding was also no longer expected along the Brunswick, Richmond, Coffs Coast, Hastings, Camden Haven, Manning, Gloucester, and Hunter Rivers.

However, flood warnings were still in place for several other catchments in the Northern Rivers, mid north coast, and inland NSW.

The SES is warning people confronted by flooded or damaged roads to stop and turn around.

Australian Associated Press