"I'm confident enough to say it's a deliberate fire, but until police have samples I can't say exactly what was used or not," he said.
The cafe was still under construction, and did not have electrics or fire alarms at the time, Mr Dickson said.
He predicted it may be "difficult" to find witnesses, because its location was removed from neighbouring houses.
The fire was "unfortunate because the new cafe wasn't far off opening", Mr Dickson said.
Cornwall Park Trust Board director Michael Ayrton said the fire about 2.30am had caused "significant damage" to the cafe, which had been due to open on November 7 after two years of planning and construction.
"We know a lot of people were eagerly awaiting the opening of the cafe and unfortunately the building has extensive fire damage throughout," he said.
"The trust is working with authorities and investigations are underway, but we're keeping an open mind at this time. We will reassess where things are at, once the investigations are complete."
The trust board spent nearly $1m on the cafe. It was hoped the cafe would generate revenue this summer to help with park maintenance.
In September, the board appointed Andrew Bell to operate the new cafe, and the former Cornwall Park Restaurant, known as the Kiosk, which is closed for renovations.
It remained unclear what or who caused the fire, which the Fire Service was alerted to about 1.40am.
Local resident Michal Petrus was disappointed to see the cafe badly damaged.
He saw the charred building while out on his morning run. "I noticed this morning that it was completely burned, to just a mere shell."
Mr Petrus said a security guard at the site confirmed the blaze happened early this morning.
"She said she'd just spoken to the operator or owner, who was on-site just to check out what had happened, and they were obviously very upset about it."
Mr Petrus said many locals and park users would be be upset. "It's a real shame. A lot of people were looking forward to that cafe opening and being used over the summer."
He said with the Cornwall Park Restaurant also closed, there would be no cafes in the park all summer.
Mr Ayrton said the trust board would consider installing temporary facilities.
He said the park received nearly 4 million visitors a year, with a spike in numbers during the summer months.
The new cafe building may have to be demolished, he said, and the restaurant further up the hill would not be open until autumn.
He said Mr Bell had already started interviewing staff and getting the cafe fitted out.
Mr Ayrton said the first priority was getting the site cleaned up. He hoped to proceed with a rebuild or renovation that was similar to the existing design.
The Fire Service said three fire trucks attended the fire.
"It was well ablaze when we got there," said Megan Ruru, northern fire communications shift manager.
Police also attended the scene, and arranged for a security guard to be stationed at the park overnight.
Road cones and high-visibility tape were added to existing construction barriers, to stop the public entering the potentially dangerous building.