The gas leak is fixed but rush-hour traffic out of Wellington is still crawling along the Kapiti Coast.
Transport authorities are asking commuters heading north out of the capital to consider staying put until after dinner as major delays continue.
Shortly after 5.30pm it was confirmed the leak was plugged and the road was opened in both directions at full speed, but heavy congestion is expected to take hours to clear.
It normally takes about 40 minutes to get from Wellington to Paraparaumu but the leak, north of Paekakariki, has pushed out travel times for those heading north. Even after six it was taking more than 90 minutes to get to Paekakariki and more to Paraparaumu from Wellington.
State Highway 1 was closed entirely for a time after the leak, which happened about 10am.
Stop/go signs were in place a couple of hours later before both lanes re-opened before 4pm, under a 30 km/h limit.
The greatest delays are between Plimmerton and Paraparaumu.
"While the road is now fully open, it will take some time to clear the backlog. So our advice is still to delay travel if possible and stay in Wellington. Another option may be to consider leaving cars in town and take the train," regional transport response team chairman Richard Hocken said.
"Paekakariki Hill Road remains closed to northbound through-traffic from Grays Rd intersection until further notice. This to ensure that traffic on SH1 can continue to flow."
Mr Hocken suggested commuters stay in Wellington until after dinner to avoid delays, expected to clear this evening.
First Gas, which owns and operates the pipeline, said there were no "immediate safety concerns" and the leak would not affect gas supply to Wellington or the lower North Island.
Its chief executive, Paul Goodeve, said a valve at the Paekakariki North mainline station sprang a leak at 10am during maintenance on the pipeline to the Wellington region.
"As soon as the gas vent occurred we reacted as quickly and as safely as possible to initiate repairs to the pipeline valve," he said.
Mr Goodeye apologised for the traffic delays.
"As soon as the incident occurred, our repair crews worked very efficiently to isolate the faulty valve so traffic flows could return to normal before peak hour traffic started," he said.
"We thank everyone for their patience while we resolved the incident."
Commuter David Nichols was caught in heavy traffic this afternoon, when his normal 30-minute run between Tawa and Paraparaumu took between 2 1/2 and 3 hours.
Travelling south to Tawa shortly after the leak was discovered, Mr Nichols was delayed slightly, but heading north traffic was pretty well stopped the whole way.
"It's never been like that before," he said.
"It ruined by day. It was a pretty bad time really and I still can't understand quite why it was so bad for all traffic both ways."
To keep himself busy Mr Nichols listened to music. He said stranded motorists were well behaved.