UPDATED 12.30pm Search teams are scouring the rain-swollen Hutt River mouth and harbour after a teenager was washed away last night.

Police said the 19-year-old and a teenage friend 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.

LandSar teams were today searching the area where the boy went missing 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 of the 19-year-old's friend could be seen frantically searching and calling out along the riverbank on Friday afternoon. 

Greater Wellington Regional Council river ranger Travis Moody has been assessing the flooding damage after last night’s rain.

He says it is as bad as the one day weather event that followed the November 14 earthquake in Kaikoura.

Moody said it was the peak of the flood event when the 19-year-old entered the water at about 10pm.

“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 it in those conditions the man would have struggled to keep his head above water.

"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 have likely been pushed out to sea with the condition the river was in.    

“I’m sure he would have turned up by now if he was okay.”

One man walking his dog said there were plenty of popular swimming spots which kids would often use along the area of the river where the 19-year-old was reported missing.

Last night's poor conditions meant they could not send up the Westpac Rescue Helicopter but firefighters helped police search the riverbank using thermal imaging hardware.

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 yesterday afternoon.

The public were warned last night to keep clear of the Hutt River because it was rising as heavy rain lashed Wellington.

Video above by Mark Mitchell / NZ Herald