Top Energy chief executive Russell Shaw said the cause had been traced to the failure of a "dump valve" designed to release energy stored in one of the 5MW generators. As a result, pressure was released through a back-up safety disc, but in a more violent manner. People nearby would have heard a series of loud bangs as pressure was released.
The sole overnight operator got "quite a shock" but followed company processes to the letter by initiating an emergency shutdown, alerting the Fire Service and evacuating the plant.
Firefighters entered the site carefully, checking for pentane as they went, and by 7.15am had found no trace of the flammable gas. There was no damage to the plant but staff were investigating why the first safety valve failed.
"That unit did not operate as planned, but that's why we have back-ups - and back-ups to the back-ups," Mr Shaw said.
The company was concerned by the incident but pleased emergency procedures had worked as planned. The Fire Service also did a "brilliant job", he said.
Muri Whenua area fire commander Allan Kerrisk said firefighters were constantly assessing the risk, and at no time were residents at nearby Ngawha Springs in danger.
Any gas that escaped would have been hot, making it rise and dissipate quickly.
About 8.30am, a column of fire trucks, including the Whangarei-based hazardous materials command unit, left the power station, along with police and St John Ambulance.
From the village, a plume of steam could be seen rising from the plant as it was powered up again.
Ngawha Springs resident Vicki Crow said she was awake around 5.30am when she heard what sounded like a valve going off repeatedly at the plant.
"And then it just gushed. It was a weird noise, it was uncanny."
She went outside but could see no light or fire glow.
"I hope they [Top Energy] have enough sense to send someone down to talk to the community. It has scared a few people," she said.
Firefighters from Kaikohe, Kerikeri, Kawakawa, Okaihau and Whangarei attended.
How Ngawha power station works
Far North lines company Top Energy operates two Israeli-built geothermal power plants at Ngawha. The first 10MW plant was opened in 1998 and the second, boosting output to 25MW, in 2009. The emergency occurred at the old plant.
The plants work by using hot water from deep underground to heat pentane, a volatile hydrocarbon in the same family as methane and butane.
The pentane is then used to drive power turbines before being air-cooled so the cycle can begin again. The pentane is used only to transfer heat.
Ngawha supplies about 70 per cent of the Far North's power needs.
The company is currently considering a major expansion which could create up to 200 jobs and make Northland energy self-sufficient. It is hoped the combination of cheap, renewable electricity with the plant's surplus heat will attract other industry such as milk processing.
A drop in power prices, however, has forced Top Energy to lay off staff and may delay the project until prices head up again.
Northland's only other power plant is Northpower's 5MW Wairua hydroelectric station near Titoki.
The last emergency shutdown at Ngawha occurred in mid-2011 when a burst safety disc, causing a release of steam, was traced to a computer card failure.
Ngawha has the country's biggest geothermal field outside the North Island's central plateau.
