The exact cause of the suspicious Mt Maunganui blaze remains unknown but investigators at the site today have found no evidence of a natural cause.

The fire on Mauao broke out shortly before midnight on Wednesday. It ripped through 800sq m of vegetation on the mountain before more than 40 firefighters, six fire engines and two helicopters brought the blaze under control.

Pumicelands principal rural fire officer Alan Pearce would not comment on the cause this afternoon while investigations continued. However, he told the Herald it was still considered suspicious.

"One thing about today's investigation -- we did not discover a natural cause for that fire, so that raises suspicion, doesn't it? It's not a spontaneous event of nature."

New Zealand fire service assistant area commander Aaron Waterreus confirms that when firefighters arrived, there were three people on the Mount.

Mr Pearce said two investigators -- one from the fire authority and one from police -- scoured the scene today.

"Following that, we got a Department of Conservation rural fire team up there and they cleared the fire grounds and dampened down a few remaining hotspots. Very dangerous work, working on that fire -- it's very steep and very rubbley."

Tauranga City Council had arranged for a geotechnical expert to investigate the scene, and that work would continue next week.

Mr Pearce said they were confident the mountain was at a "safe stage" and the council had now opened up the lower tracks.

The upper tracks remain closed and it would be up to council when they would reopen.

There have been reports of a flare in the area before the fire. Police investigating the blaze have called for any information from the public.