SES southern incident controller Ben Pickup said SES members provided assistance in the rescue and recovery efforts with police to locate and retrieve the vehicle and the child.

On Saturday at 3.20 pm, SES and police rescuers found the five-year-old boy inside the submerged Toyota Hilux discovered roughly eight metres below the surface of the swollen Genaren Creek at Tullamore.

The child was part of a family of five traveling in the white Toyota Hilux along the McGrane Way from Dubbo back to Tullamore when, in the dark of night at around 8pm, they attempted to cross a creek they had navigated safely earlier that day.

The floodwater levels had since risen, and the ute was swept off the road.

The mother and father in the car were able to release two of the children from their harness seats in the car and they clung to trees for hours before being rescued.

Tragically, the five-year-old boy was unable to be released from the car in time.

"Our team was part of the assistance provided for the police and the whole incident was traumatic for those involved," Mr Pickup said.

"We are offering assistance, peer support, and counselling.

"It's always distressing in these types of incidents but our volunteers are experienced in handling these incidents."

Police said on Sunday an investigation into the circumstances surrounding the incidents was underway. A report will be submitted to the coroner.

The surviving family members were taken to Dubbo Base Hospital for observation taken by ambulance from Tullamore and received medical treatment.

The NSW premier has said the death of the young boy was "incredibly tragic" while urging people to heed warnings.

Dominic Perrottet said it was another tragedy brought on by the floods NSW had been experiencing for the last two years.

"I extend on behalf of everybody across our state, our thoughts and prayers and hearts go out to the family," the premier said on Sunday.

"Because it is so saturated and all the river systems are at capacity, even a little bit of rainfall could impact or change the flooding conditions," senior meteorologist Miriam Bradbury said.

The flood impacts were likely to linger for weeks, if not months. There was a low threat of severe thunderstorms but if they did form, they would bring heavier rain.

Many regional roads remain closed due to flooding. Motorists are urged to check www.livetraffic.com prior to travelling.

Low to moderate rain was expected across inland NSW until Wednesday before the weather system is expected to move offshore, bringing moderate rainfalls across the state's coastline through to the weekend.

Two NSW towns are likely to be isolated for days as the flood emergency continues.

State Emergency Service workers performed six rescues and answered 112 calls for help between 6am on Saturday and 6am on Sunday.

The towns of Warren and Wee Waa, in the state's north, are cut off by road and expected to remain that way for days.

Emergency services are using aircraft to delivery water, medicine, food and other supplies to residents.

The town of Gunnedah continues to be impacted by major flooding as of Sunday morning, however water is slowly receding.

The Namoi River peaked at the major flood level of 8.24 metres on Saturday and SES workers were on the ground to assess the damage as it became clear.

Authorities have reinforced safety warnings in the wake of the five-year-old boy's death, renewing calls for people to take the dangers of floodwaters seriously.

SES spokesman Greg Nash saying a river or creek's conditions could change between morning and afternoon.

"We have some very resilient communities and some communities that are very much aware of their flood risk," Mr Nash said on Saturday.

"(However) we want people to drive to the conditions, and if you find a flooded or a damaged road as a result of flooding, to stop, turn around, (and) find another away - it's not worth the risk to push on."

There are concerns more inland catchments could spill over, with many dams already at capacity.

In the 11 days since the flooding began, emergency services fielded more than 820 requests for assistance in NSW, which included nearly 60 flood rescues.