The restaurant suffered "major damage" and so did the office block at the rear of that building, Swinburn said.
It was an older, two-story building and the flames had caught through the roof of the structure, he said
"So it was difficult to tackle but a good effort from all of the brigades.
"There were three other commercial premises attached and they were all saved."
The response time was really good, the neighbouring brigades were there within five minutes, he said.
Smoke was initially seen coming from the restaurant, before it spread to an electrical wholesale business.
The fire was nearly extinguished by 8.30am but a fire investigator would remain on site today as the cause was yet to be determined.
Detective Sergeant Kirsten Norton said a total of four businesses had been affected due to the fire spreading.
"The buildings have suffered substantial damage," she said.
"Early indications are that the fire was deliberately lit and police are continuing with a scene examination this afternoon.
"We would like to hear from anyone who was in the Tanui St area between 3.30 and 4.15am and may have seen any suspicious activity."
People can call Greymouth Police on (03) 768 1600 or provide information anonymously via Crimestoppers on 0800 555 111, she said.