The cause of a gas explosion in south Auckland which claimed the life of a 48-year-old woman on Saturday is still unknown, despite police completing their forensic examination at the scene.

Inquiries were continuing at the Onehunga site, but nothing obvious about what could have caused the explosion had been revealed, a police spokeswoman told NZPA tonight.

"Police have finished their forensic examination, which involved taking photographs and gathering evidence at the scene. But investigations are continuing into the cause of the blast," she said.

Firefighters completed their scene examination yesterday, and did not return to the site today.

Watercare had begun preliminary work at the scene by flushing out the pipes with water and carrying out gas checks.

It was due to resume work on fixing the damaged pipe-line this morning but this had been delayed until tomorrow, according to HEB construction, which has the contract for connecting the water pipes.

Philomen Gulland, a Canadian mother-of-two, a maintenance planner for Watercare Services, was killed when the watermain pipe she entered exploded.

Fire investigators said that methane gas had been found in the pipeline but the cause of the explosion still had to be determined.

Fire Service area commander Murray Binning, officer in charge at the scene, said investigations were continuing to investigate how the methane gas got into the pipe.

"It's a naturally occurring gas, and we are not sure of the source at this stage," he said.

"We still have no idea what started the explosion."

A camera flash in the excavation area, was one possibility, he said.

Ms Gulland moved to New Zealand six years ago with her then husband and children Emma and Cameron, and police had spoken to her family, district commander Superintendent George Fraser said.

"It's an exceptionally difficult time for them and we are providing the necessary support," he said.

Watercare Services chief executive Mark Ford said yesterday it was "a very dark hour for us".

"... many of the crew on site are like family to us, and my heart goes out to all of those affected by what has happened."

Three other injured Watercare workers remained in hospital today.

Ian Winson, 47, a network engineer for Watercare, was in a "seriously ill but stable" condition in the intensive care ward at Auckland Hospital. He lost both his legs in the explosion.

Harry Barnett, 56, who supervises civil maintenance for the company, and Mathew Jamieson, 25, a drain layer, were both in stable conditions.