Police have discovered the body of a boy believed to be an 11-year-old boy who may have been sucked into a drain while sliding in the wet with friends in the NSW Illawarra region on Thursday.

Ryan Teasdale was last seen riding a bodyboard with his brother and friends in Riley Park at Unanderra on Thursday afternoon as torrential rain swept through the area.

Police say the body was found in a creek bed in Unanderra, and the family was being supported.

The area where Ryan was boogy-boarding is popular with local kids in wet weather.
The area where Ryan was boogy-boarding is popular with local kids in wet weather.

The area where Ryan was boogy-boarding with his brother and friends is a popular play spot for local kids in wet weather. The children were being supervised by adults at the time.

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Rescue workers were focusing their search on the area's drain network, using cameras to look for the boy inside as rain returned to the east coast of the state today.

Briefing media on the search for Ryan, police said if the boy had been sucked into a stormwater drain system, which connects to Albert Creek and leads into the Harbour, he could have been transported to "countless" locations.

Ryan's brother raised the alarm about 4.30pm when he couldn't find him, beginning a frantic search.

A local man who was at the scene expressed his disbelief to a Channel 7 reporter, saying he just couldn't fathom how the boy had disappeared.

"We were all boogy boarding here and all and it's what kids always did around," he said.

"That's why I can't get it. There were how many grown people here?"

Ryan was last seen wearing blue boardshorts without a shirt and holding his blue bodyboard.

NSW Police will hold a briefing on the search on Friday morning.

The Illawarra region has been pelted with rain in recent days, with SES warning residents to take care in flood waters.

"We had almost 200mm of rain ... and that really caused some danger in terms of flash flooding. We did 40 flood rescues in a three-hour period yesterday, largely they were all drivers driving into flood waters," NSW SES Commissioner Mark Smethurst told ABC on Friday.

"It's a bit crazy. People are risking their own lives and that of the volunteers."

Mr Smethurst warned the flood risk could continue over the weekend.

"We're predicting rain around the 50-100mm on both Saturday and Sunday and those areas that have already hit total saturation."