An Auckland vape shop was destroyed overnight when a car was deliberately driven through the front of the building and set alight - while people were asleep one floor above.

It was the second suspicious fire this weekend at the VapeBox shop on Manukau Rd in Epsom, police say.

Specialist fire investigators are at the burnt-out shop this morning to determine the cause of the blaze, while police are reviewing CCTV footage and looking for those responsible.

Fire and Emergency NZ was called at 3.05am today to a report of a structure fire in Epsom with people still in the building, northern shift manager Daniel Reilly said.

Thirteen crews were sent to fight the blaze, he said.

When fire crews arrived the two-storey building at 471 Manukau Rd - the premises of VapeBox - was "well involved" in fire.

"The crews have got here, found a very intense working job and have got into it really quickly and knocked the fire," Onehunga senior station officer Jason Cameron told media at the scene.

The fire had been extinguished and crews were working to establish the building's integrity.

There was sleeping accommodation above but everyone had been accounted for, Cameron said.

"We're just working to make the scene safe for police to do their work."

St John was called to the scene at 4.42am, with three ambulances attending. One patient was treated in a moderate condition and two in a minor condition; none were taken to hospital.

Detective Senior Sergeant Kathryn Bostock, manager of the Auckland City crime squad, said police were investigating two suspicious fires at the property.

This morning's fire followed a separate fire around 3am on Saturday, when the shop front was set alight.

The occupants of the residential property above the store were home at the time of both incidents, Bostock said.

"While they were fortunately not injured in either incident, police are mindful that the outcome could have been very different, and we are taking this matter very seriously."

"We would like to hear from anyone in the Manukau Rd area around the times of these two incidents, who may have seen something which could assist our enquiries," Bostock said.

Anyone who can help should call 105 and quote file number 191117/1234.