A fire that ripped through Wanaka earlier this month was caused by a local discarding ashes from an outdoor fire.

Photo: FENZ

More than 40 firefighters and eight helicopters with monsoon buckets battled the blaze on 3 January. It took two days to bring it under control.

A Fire and Safety New Zealand investigation took two weeks to uncover the cause of the fire.

Fire and Emergency region manager rural Mike Grant said the person who caused the blaze had been allowed to light a fire.

The person had taken steps to ensure the ashes from that fire were disposed of in a safe way, Mr Grant said.

"Basically, the person did all the right things.

"They put the embers in a bucket, then let them cool for two days before discarding them in a pit, but because of the dry conditions, it just wasn't enough," he said.

"It might look like they're cool on the surface, but underneath they're often still hot and that's all it takes, given the dry and the heat are so extreme at the moment."

Fire and Emergency principal rural fire officer Graeme Still said that anyone discarding embers should put them in a bucket, and douse them with water, stirring them occasionally and letting them soak for several days.

"Every year we get several calls to fires that start from ashes, so it's a good reminder to doubly, even triply, check them," he said.

"Even best practice can catch you out when the fire danger is as heightened as it is now.

"There's absolutely no moisture in the ground and things can escalate pretty quickly."

Firefighters had been going back to the site of the fire every day, dampening hotspots and making sure there were no flare-ups, Mr Still said.

It was lucky there had not been a lot of wind, otherwise the fire could have caused a lot more damage, he said.