Police are searching for missing 11-year-old boy Ryan Teasdale in waterways.

Police say they are fearing the worst for 11-year-old Ryan Teasdale, who went missing in floodwaters in Unanderra on Thursday as heavy rain caused chaos in the Illawarra.

Key points: Police say several children were playing in floodwaters in Unanderra on Thursday

Police fear Ryan Teasdale, 11, has been swept into a stormwater drain

Heavy rain caused chaos in the Illawarra, with roads cut and trains cancelled

Police said Ryan was last seen riding a boogie board in heavy rain in the company of numerous other children about 4:30pm in Riley Park.

Inspector Brian Pedersen, from the NSW Police, said the search started in the park but moved to waterways.

"We have strong fears he may have gone into a stormwater drain," he said.

"At the moment we have rescue squad and water police checking our waterways and stormwater areas for any sign of him."

He said the area's stormwater systems were flowing "extremely quickly".

"Of course we wish that it does come to a happy conclusion," he said.

"The situation with the torrential rain and the water run-off we've had, we are fearing the worst unfortunately."

Witness said 20 kids riding boogie boards

One witness said a number of children were riding boogie boards down a hill that connects to a stormwater drain.

"A whole lot of kids turned up with their boogie boards — about 20 of them —[and there were parents up there as well," said Andy, a woman who lives across the road from the park.

"They were just riding the water down the hill and it does connect up to the drain if they go far enough…the water flows down the hill and into the drain," she said.

Andy realised there was a problem about 7:00pm on Thursday.

"I went over and talked to the mum and apparently her son had gone missing and it wasn't like him.

"It was him and his brother down here, and his brother went running home saying his brother had disappeared."

Andy said the mother was "quite devastated".

"I have an 11-year-old myself. We don't let our kids play in the park over there like that. I'm kind of glad that we never have."

Between 9:00am and 8:00pm, 201mm of rain was dumped on Macquarie Pass, while Robertson (189mm) and Wollongong (149mm) saw heavy falls.

Another witness said he noticed a commotion about the missing boy at about 6:45pm and spoke to Ryan's mother.

"She was quite devastated and very worried," the witness said.

"A whole lot of search crews and police turned up and they searched the drains."

Police from Lake Illawarra Local Area Command, Police Rescue, and SES volunteers are searching for Ryan, who was last seen wearing blue board shorts and no shirt.

He also had a blue body board, that has not been located.

This morning, dozens of police officers and SES volunteers are preparing to search a storm water drain not far from where Ryan disappeared.

The New South Wales State Emergency Service (SES) said they successfully rescued 43 people last night, and responded to more than 700 calls.

Strong winds and rain affected much of Sydney's Illawarra region while roads were cut and trains were delayed due to the downpour.

The rain forced the closure of several roads in the area, including the Princes Highway, which has since reopened, and Illawarra Highway.

Two passenger trains had to stop due to flooded tracks.

The Bureau of Meteorology (BOM) has issued a severe thunderstorm warning for heavy rainfall for people in the Illawarra and parts of the south coast.

Locations that may be affected include Wollongong, Nowra, Port Kembla, Albion Park, Huskisson and Ulladulla.