A passerby, who didn't want to be named, saw smoke coming from the car and ran to help the mother.
"The car seat was a little bit stuck; the release button wasn't releasing immediately," she said. "There was visible flames in the car close to the little girl. If it had have been another few minutes it could have been quite a different outcome."
Mr Davies said the blaze was put out by someone at the scene with a fire extinguisher before the Fire Service arrived, and the children did not suffer burns or smoke inhalation.
National manager of fire investigation Peter Wilding said children playing with lighters was a problem "close to [his] heart".
This was the third fire that had been started by children left unattended in a vehicle with lighters or matches this year, he said. In every case the children were plucked to safety by someone who'd seen fire.
"In all of the last three cases there just happened to be a bystander walking past that could take action," Mr Wilding said. "The chance of those kids being alive if [bystanders] had been waiting for us to arrive would have been pretty slim."
He said no lighters were childproof.APNZ