Seven people injured in a gas explosion at a service station south of Hobart have been released from hospital.

Witnesses say it appears a gas bottle on the back of a ute exploded, but police say it is too early to determine the cause.

Power to the site was turned off as investigations began and emergency crews cordoned off the site.

It ripped a two-metre hole through the roof of the station, damaged several cars and injured people both at the service station and nearby.

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Inspector Ian Lindsay says the injuries were mostly bruising and cuts.

"There is no doubt that we can be thankful that there weren't some more significant injuries considering the place where the explosion occurred and the extent of the debris," he said.

The fire service and police are reviewing CCTV footage of the explosion.

The service's Rod Sherrin says the investigation is focussing on the ute.

"What we can tell is that the driver left the ute to come and pay for his purchase of fuel and he turned around to lock his vehicle with his central locking and when he pressed the button that's when the explosion occurred," he said.

Witness Gabrielle Synnott was next door and said she heard the explosion.

"There was about five vehicles in the bay at the time - they've all lost their windows," she said.

"The main window to the service station has shattered.

The explosion scattered debris around the site. ( Carla Johnson (Twitter) )

"Everyone is pretty good. There's only one cut to a head, about three people that can't hear, one lump on the head, but mainly shock.

"So everyone is actually good, which is pleasing."

Jess Reaney was working at a bakery opposite the service station when the blast occurred.

"I was just at the counter and I heard a bang and I looked up and stuff was just flying through the air and hitting our windows."

Police said there were no road closures.