Firefighters are anticipating they will still be fighting the Matakana Island's blaze towards the New Year, as it continues to spread underground.

The 'tumultuous” fire first broke out on the Panepane Point side of the Bay of Plenty island on December 13, covering around 19 hectares of land and producing large plumes of smoke over Tauranga.

Once the flames were extinguished, firefighters spent the rest of the week dampening down hot spots, but on Saturday it flared up again.

Now, Principal Rural Fire officer Steve Webb says crews, assisted by helicopters and drones, will be working beyond Christmas until they are 'satisfied there's no threat”.

'While there is little smoke in the atmosphere now, the fire is still burning deep underground.

'We have between 60-75 crew members on the island each day and around 20 overnight.

'We will be working beyond Christmas, until we are satisfied there's no threat to the forest and the people on the island.”

He says the source of the flare up was either due to underground burning or a toetoe bush that flicked embers towards other vegetation.

The island has a lot of pine trees with intricate root systems and a large duff layer, a decomposing organic material layer, that when dried can easily ignite.

That, with the combination of sand has created a matted material under the surface fuelling the fire to now double in size, covering around 27 hectares of land, Webb says.

The depth of the underground fire varies depending on the size of the duff layer.

'It's giving us a lot of problems.

'Every night at 4am we have a drone team on site, and they have very sophisticated infrared cameras, and they fly the area to map where any hotspots are, so the crew know where they are the next day to put them out.

'It takes hours, but if we don't do that the fire could spread underneath a road for example and pop up on the other side of the road, with the unburnt fuels and start the fire all over again.”

He said it's not uncommon to see fires flare like this during the dry season, but it's a bit early in the season to see one spread as quickly as this fire has.

The cause of the fire is still under investigation, but 'don't normally start by themselves in New Zealand”.

'It's being investigated thoroughly, but we have found some evidence of people in the area.

'It looks like someone was in the area, someone potentially doing some sort of recreation, like smoking or a campfire.”

Webb says the fire will continue until they either get all the underground hotspots identified or 'a heck of a lot of rainfall comes”.

Fire have been working closely with local Iwi on the Island. Matakana Island locals haven't been evacuated and aren't under 'any eminent threat”.

Webb would like to thank organisations such as the Department of Conservation, the Forestry community and Fire volunteers who have pulled extra resources together to assist with the blaze.

-Stuff/Sharnae Hope.