He was heard to say he had "had enough" and would make the car disappear.
A witness, Andrew Gray, filmed the explosion but told One News last night he did not know what Polenski was about to do.
"I was in shock, I thought 'what's going on here' [but] kept filming and once I realised what was going on I ran off to try to find a fire hose."
He originally thought the man was a street performer which is why he started filming. He said if he had known the truth he would have tried to stop Polenski.
Detective Sergeant Michael Priston, of Wellington police, said Polenski's actions were dangerous.
"These actions clearly endangered members of the public who were walking in this busy mall area."
Officers had spoken to 16 witnesses but asked for people who had footage of the explosion to come forward.
Mr Priston said police believed an accelerant was used to start the fire but there was no evidence that any explosive device was used.
Polenski was known to police and appeared in court last week on charges of threatening behaviour, thought to be related to Saturday's protest.
