The head of the Auckland Council-run water company says he believes reticulated methane gas was the source of a fatal explosion.
A 48-year-old Canadian engineer was killed on Saturday and six other people injured during work to connect two parts of a water mains pipe in Onehunga.
Watercare chief executive Mark Ford said on Wednesday he would be surprised if the source of the methane was from anything other than the reticulated gas supply.
Mr Ford says staff on the site could smell the odourant that is put into the city supply.
He admits he is no expert, but says Watercare is not investigating the possibility of the methane coming from a natural source.
However, gas company Vector says there were no signs of a loss of pressure in any of its systems on the day of the explosion.
It says it is monitoring levels, as it always does, but is relying on a Department of Labour investigation to determine what caused the blast.
Watercare says welding to connect the pipes was carried out about 11pm on Tuesday and went to plan.
Meanwhile, the man most severely injured in the blast, Ian Winson, remains in a serious but stable condition in hospital on Wednesday.
Another man is in a stable condition and the third hospitalised man has been discharged.