Wellington commuters face long delays with all roads out the city congested after a landslip in Ngauranga Gorge blocked southbound traffic on State Highway 1 this afternoon.

Photo: Twitter: NZTA

The slip has blocked the southbound lanes of SH1 north of Wellington, and Newlands, Johnsonville and Tawa on-ramps are closed.

However, all northbound lanes were now open but it would take some time for congestion to clear.

NZTA regional transport systems manager Mark Owen told Checkpoint with John Campbell it was "very-heavily congested" right across the whole network as people tried to take alternate routes home.

This was particularly the case on SH58 over Haywards Hill and on parallel routes running through Tawa and Johnsonville.

An RNZ reporter said traffic in Wilton, Wadestown, Khandallah and Ngaio was also at a crawl.

Mr Owen said he hoped to have two of the three southbound lanes open by 6am but this would be subject to final safety assessments.

Photo: RNZ / Colette Jansen

The Fire Service were sluicing water onto the slip to remove any loose material, and geotechnical engineers were assessing the area.

A 50km/h speed limit would be in place in the area for at least the next few days, Mr Owen said.

It was unusual that the slip came down in fine weather, he said.

Photo: RNZ / Michael Cropp

Wellington City Council said there were extensive delays in Ngaio, Khandallah, Johnsonville and Tawa as traffic was diverted away from the gorge.

It said motorist should expect "major delays" and to put off their commute and avoid travel if possible.

Metlink said Mana/Newlands services between the city and Johnsonville were suspended in both directions because of the slip.

It said passengers were advised to take the train to Johnsonville and then transfer onto buses.

Photo: RNZ / Colette Jansen

Wellington Live posted a video to Facebook from Asispal Sandhu, which it said showed the moment the landslide happened: