A friend of a 19-year-old missing in the rain-swollen Hutt River was frantically searching and calling out on the bank this afternoon.

Search teams are scouring the rain-swollen Hutt River mouth and harbour after the teenager was washed away last night.

Police said two teenagers went swimming in the flooded river around 10.30pm near Belmont School.

Sergeant Anthony Harmer said when one of the teenagers got out of the water he could no longer find his friend.

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Greater Wellington Regional Council river ranger Travis Moody said the river was at its peak when the 19-year-old entered the water at about 10pm.

Source: Carey Griff

"It gets quite ferocious even in a smaller rain event like that. Any time the river's up you don't want to get in the water.

"It's a raging torrent basically. You'll have big boulders underneath the water moving along with that flow."

Moody said the man would have struggled to keep his head above water in those conditions.

"Even if you did get to the side, you'd have to deal with a whole lot of vegetation.

"It's pretty easy to get pulled under when it's like that, poor guy."

Moody said the man would probably have been pushed out to sea.

"I'm sure he would have turned up by now if he was okay."

LandSAR teams were today searching the area where the youth disappeared as maritime police searched Wellington Harbour and the river mouth.

The police dive squad was on standby but conditions were still too treacherous for the team to enter the water after last night's heavy rain.

This morning the river was still swollen and had breached its banks in some parts.

One man walking his dog this morning said there were plenty of popular swimming spots along the area of the river where the 19-year-old was reported missing.

A police officer and her dog search the bank of the swollen Hutt River at the Belmont Domain, near where a swimmer went missing. Photo / Mark Mitchell
A police officer and her dog search the bank of the swollen Hutt River at the Belmont Domain, near where a swimmer went missing. Photo / Mark Mitchell

A NewstalkZB reporter said the current was flowing swiftly where the two youths entered the water for their ill-fated late-night swim.

MetService meteorologist Brian Mercer said parts of the Wellington region, including Lower Hutt, recorded around 10mm of rain hourly on Thursday afternoon.