Fire cause remains mystery (+video&pics)

What sparked the Sulphur Bay blaze remains a mystery, with fire investigators unable to pinpoint the cause.

However, police are yet to rule out the possibility it was intentionally lit.

Specialist fire inspector Stuart Bootten said the investigation into Sunday's fire had ended.

"We have determined it as undetermined. We just can't pinpoint the cause, there's just too much destruction. We know where it started but don't know how it started," Mr Bootten said.

Mr Bootten said the information gathered by his team had been passed to police.

Image 1 of 13: Scrub fire in Rotorua. Photo/Ben Fraser

Inspector Stu Nightingale said Rotorua police were investigating the fire.

He said it would be treated as suspicious while investigators tried to determine whether or not it was intentionally started.

Meanwhile, the Te Ara Ahi Cycle Trail and adjacent walking tracks that were closed as a safety precaution reopened to the public today.

Rural Fire and New Zealand Fire Service crews had been on site mopping up and monitoring conditions since the big scrub blaze and sulphur deposit fires broke out.

Rotorua Lakes Council said in a statement the last hot spots were under control and people could again use the cycle trail and walking tracks.

However, it warned people to keep to the defined tracks and not ride or wander off into other areas on the sulphur flats where the risk of unstable ground remains.

The fire started about 3.30pm and quickly spread, with flames dispersing 500m to 1km gutting bush and smaller trees. Smoke could be seen from all over the city, with reports of ash reaching Lynmore. It took more than four hours for 10 fire trucks and two helicopters with monsoon buckets to contain the fire.

Anyone with information can call police on (07) 348 0099.