Incense sticks being used to smoke out moths have been blamed for a blaze that caused $750,000 damage to a North Adelaide townhouse.

The fire started just before 6:00pm (ACDT) on Saturday and sent black smoke across surrounding streets and parts of the city.

About 50 firefighters stopped it from spreading to adjoining homes on Finniss Street, which runs parallel to Melbourne Street.

Firefighter Chris Smith said the blaze, which gutted the house, started in the garage and flames quickly moved into the roof.

About 50 firefighters stopped the blaze from spreading to adjoining homes. ( ABC News: Nicola Gage )

"The front part of the house has been saved, but there is considerably damage to the back, two-storey portion," he said.

"The nature of the building was such that the floor and the roof collapsed rather early in the incident, but the design stopped the fire from spreading to the neighbouring property."

Mr Smith said an internal firewall was crucial in containing the blaze.

The five people inside the home at the time were alerted by a smoke alarm, and one of the residents was treated for smoke inhalation.

"It appears, on initial investigation, that they'd been using some incense sticks in the garage to try and move an infestation of moths," Mr Smith said.

Neighbour Trevor Stirling, whose house suffered smoke and water damage, said he initially feared it was his house that had been gutted.

"We had just arrived at a wedding reception and 15 minutes into it, we were told all the alarms had gone off. That was all we knew," he said.

"When I drove around the corner and I saw the whole street blocked off ... it was pretty scary.

"The firies said that their main focus was to keep our house safe."