Photo: 123RF
A sudden spike in hydrogen sulphide in pipes at Mangawhai north of Auckland has prompted safety warnings.
The Kaipara District Council's readings at pump stations have gone from zero to five parts per million, to more than 200 parts per million this month.
Inhalation of hydrogen sulphide can be fatal.
The council believes the high levels have stemmed from unused house connections, where wastewater has stagnated in pipes without flowing through to the treatment plant, and produced the gas.
Infrastructure Services general manager Anin Nama told RNZ the gas sometimes had an odour.
"A rotten egg smell. It's similar to Rotorua, the hydrogen sulphide that's given off there through the geysers."
He said residents should flush water through their drains, toilets and showers this weekend, and they should double-check pipes under sinks, showers, baths and tubs for blockages and leaks.
Anyone who could smell the gas should ventilate their homes by opening windows.
Next week the council plans to start chemical dosing through the piping network, "which will reduce the [gas] levels significantly".
"We're also then looking to undertake a study, a more detailed study in terms of what's causing this issue."