Police are appealing for a tractor driver and two cyclists seen in the vicinity of the Arthur's Pass blaze just after it started yesterday to contact them.
The fire started about 2.30pm yesterday on the Flock Hill Station side of State Highway 73, opposite Craigieburn Forest Park.
It has been described as being the size of 300 rugby fields and there had been a warning it could threaten native species and recreational facilities such as skifields and lodges.
However, firefighters had managed to contain it this afternoon.
Detective Inspector Greg Murton said the blaze appeared to have started in four separate locations along SH73.
"A member of the public has provided police with dash-cam footage which shows two cyclists close to where the fire is believed to have started," he said.
"We are keen to talk to them as they are key witnesses and may be able to provide us with information that can help with our inquiry. We understand that a tractor had also recently driven past the area and are keen to talk to the driver also.
"The fire could have been deliberately lit or it could have been caused by a passing vehicle, and that's why we wish to speak to the tractor driver who was also in the very close vicinity at the time, going up that hill.
"But we certainly do need to talk to those two cyclists because they are so close to where it started and when it started."
Mr Murton said the cyclists and the tractor driver, or anyone who had information on the cause of the blaze, should contact police on (03) 363 7670.
Department of Conservation (DOC) spokesperson Craig Alexander said lighter winds and lower temperatures had helped contain the fire's spread today and up to 50 residents in the small settlement of Castle Hill had been told they would not need to evacuate their homes.
However, conditions were still too dangerous for ground crews to fight the blaze and it was being fought from the air.
Mr Alexander expected at least 35 ground-based firefighters would be able to start fighting the blaze tomorrow, providing weather conditions remained favourable.
A DOC investigator is trying to determine the cause of the blaze, which was still unknown.
DOC fire officer and incident controller Bruce Janes said the threat to native species and facilities in Craigieburn Forest Park and Castle Hill Village had been greatly reduced and that while forecast conditions for tomorrow were favourable, it would be a "long, hard, hot day".
The National Rural Fire Authority has organised a team of 70 firefighters from as far afield as Whangarei to bolster ground crews tomorrow, and a fire crew will remain on watch tonight for flare-ups.