The most recent incidents involved two vehicles being set on fire in a car park on Grafton's Park Rd about 9.15pm on Friday.
Detective Senior Sergeant Martin Friend said one vehicle had minor damage while the other was badly damaged.
It followed three other arson incidents around Auckland Domain.
One happened about 9pm on Wednesday, another about 10pm on Thursday and the third involved two cars on Stanley St about 5pm on Friday.
"The vehicles suffered varying degrees of damage," Friend said.
"These fires have caused a lot of needless disruption to the lives of the victims, and they're understandably annoyed at what has happened. We're also concerned that these fires could have put people's lives at risk."
It follows a flurry of vehicle fires further north in the Whangārei suburb of Tikipunga.
Whangārei police are now investigating five car fires this month alone.
Detective Sergeant Pieter Serfontein said all the fires had started before dawn.
"I'm asking those that live in Tikipunga, or those passing through, to be extra vigilant around this time period," he said.
"We need to hear straight away if you see suspicious activity taking place. Whether you're walking the dog, going for a walk or heading off to work - we need to hear from you."
Friend said police wanted to hear from anyone who may have seen any suspicious behaviour around Auckland Domain at the times the fires were reported.
