"This fire appears to have started in four separate locations along the east-bound bank next to SH73 yesterday," said Detective Inspector Greg Murton.
"A member of the public has provided police with dash-cam footage that shows two cyclists close to where the fire is believed to have started.
"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."
Mr Murton said there had been considerable damage to surrounding paddocks and bush.
Anyone with information on the fire can phone police on 03 363 7670 or provide information via Crimestoppers on 0800 555 111.
A skeleton ground crew worked to stop the fire spreading last night.
The Department of Conservation had now taken over the fire fighting efforts.
Castle Hill Community Association chairman John Reid said the local community was prepared for evacuation.
An emergency response team has been set up and visited those who have stayed behind.
There are around 30 people left, including visitors, at the village which has 125 houses and around 20 permanent residents.
However, Mr Reid said he drove near the site of the blaze this morning and thought the chances of being evacuated were now remote.
"There's no wind and it's a calm day up there, thank goodness," he said.
"If there had been a big nor'west wind blowing there would've been problems."
There were no houses or farms currently in danger, he said.
It's been an extremely hot and dry summer for many parts of the South Island.
The scrub in the Arthurs Pass area is parched and brown.
Mr Reid, however, has lived in the area for more than 40 years and it's the first major scrub fire he can recall.
DoC: Evacuation plans in place
Department of Conservation fire technical support officer Craig Alexander told Radio New Zealand's Morning Report earlier today that the fire was based in the Flock Hill station in the Craigieburn Forest Park, and had yet to be contained.
Stock had been removed from the station overnight and an evacuation plan was in place for the nearby settlement of Castle Hill Village, he said.
"We've had a huge number of fires across the region so far this year, exceeding a number of other years.
"It's just been a matter of time before something like this might have happened."
Mr Alexander said they hoped to have the fire under control as soon as possible, helped in part by weather forecasts which had predicted temperatures to drop, and the large amount of air support fighting the fire.
"There is the potential to have some drizzle and light rain this evening, but the temperature and the relative humidity is forecast to be lower today so it lessens the explosive intensity."
Due to winds in the region, fighting the fire yesterday was like "chasing a rampaging bull", Mr Alexander said. "It was moving from one direction to the other."
A lack of available water in the region had also proven problematic, he said.
"Streams have dried up so we're trying to find decent water points for that air attack."
Due to the extreme conditions, fire crews were unable to fight the fire from the ground, Mr Alexander said.
"Safety is number one for our personnel - we want to make sure everybody comes home safe."
