A fire which gutted a Greymouth restaurant this morning is being treated as suspicious.

Photo: RNZ / Richard Tindiller

The fire broke out in a restaurant on Tainui Street about 4am and spread to other tenants in the large commercial building.

No one was in the building at the time of the blaze.

Police said early indications were that the fire was deliberately lit.

Police are continuing with a scene examination this afternoon, but they would like to hear from anyone in the Tainui Street area between 3:30 and 4:15am.

More than 60 firefighters and nine trucks, which came from as far afield as Hokitika and Blackball, battled the blaze for much of the morning.

They were unable to save the restaurant and some neighbouring law firm offices were severely damaged.

Three other neighbouring businesses sustained minor damage.

Greymouth's chief fire officer Lee Swinburn said the type of building made the fire hard to control.

"The inside of the restaurant was pretty well totally gutted. The shell is still standing but the nature of the structure and the age of it meant it was a very difficult fire to tackle."

Lee Swinburn said the fire spread through the roof cavity, so the law firm next door had a lot of damage to the ceiling and water damage.