Emergency services were called to Domaine Restaurant, on Victoria St, just before 2.30am today. The fire started at the back of the building.

Domaine owner and head chef Jeff Dunstan has no idea what caused the fire that destroyed "his baby", but is already thinking about reopening.

The fire broke out about 2.30am on Friday, just hours after he and the manager left the Victoria Street eatery.

"I left at around 9.10pm.

CHRISTEL YARDLEY/FAIRFAX NZ Fire crews shut down part of Victoria St to fight the blaze.

"The manager closed up at around 10.10pm."

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CHRISTEL YARDLEY/ FAIRFAX NZ What remains of Domaine Eatery on Victoria St.

Dunstan was unsure what could've started the fire.

"The guys checked that all the [gas] nobs and dials were turned off, and the gas and so forth, so it seemed like everything was in order, but who knows?

"That's my little baby. You put in over 100 hours a week and you don't see your family, so yeah.

"It's gut-wrenching."

Dunstan has been a part of the restaurant for four years.

"[I'll] definitely look at reopening again. I love hospo, I love being a chef and I love pleasing people."

The fire ripped through the popular central Hamilton eatery, eventually sending flames through the roof.

The first fire crews were called shortly before 2.30am on Friday.

About 40 firefighters used "aggressive" tactics to tackle the blaze, which left the roof at risk of collapsing and water damage in surrounding stores.

It took fire crews at least an hour to contain the fire and several neighbouring upper-level apartments self-evacuated. Nobody was injured.

Six of the 13 fire crews from around Hamilton and Ngaruawahia were still at the scene at 6am.

Fire Safety Investigator Peter Hallett said at 1pm on Friday the cause of the fire remained undetermined.

"We are still working through the debris - the kitchen is a place of interest, but we are still trying to clear a path into the building."

The last fire truck left at midday after dampening down remaining hot spots.

Part of Victoria was cordoned off to allow fire trucks equipped with a cherry picker to assess the damage.

The owners of the building, Brian and Raywn Milicich, were woken by the fire crews as they slept in their neighbouring flat.

"I could hear the sounds of the hoses being pulled, got up and saw all the lights flashing," Raywn said from the street at dawn on Friday.

"I opened up the windows to have a look and there was smoke everywhere."

The couple got out through the back door and watched the restaurant become engulfed in fire from the street.

"We were worried about fire on both sides, but the fire walls stopped the fire from spreading."

Brian kept thinking, how bad is it going to be?

"We were glad to see the people from the flat at the back of the building had got out safely."

He had some blankets the car, so they were able to keep warm in the early morning chill.

They were still in shock at the loss of the restaurant they originally started in 1998.

"Apparently the roof has fallen in, but we have no idea of the damage yet. It was good that they contained it and it didn't spread to the side buildings."

Waikato Assistant Fire Area Commander Darryl Papesch could see flames from the western end of Victoria St when he arrived at 3am.

"You could see the flames out the roof, straight up into the sky. The whole place was going.

"The building was fully smoke logged and we were just about to make entry when fire raced through the whole building," he said on Friday morning from the Fire Service's Command Unit set up on Victoria to manage fire operations.

Fire was burning fiercely in the kitchen at the rear and quickly spread to the street.

"It was fully involved really quickly - it vented through the roof and once that happens, it's away - all the oxygen gets in, the fire just carries on."

Fire spread under the flashing of a neighbouring store, but fire walls stopped it from spreading.

Papesch said fire crews worked to contain the blaze while checking there was no one inside the building and neighbouring flats.

"At the same time, the guys did some aggressive firefighting."

Conditions were hot, smoky and intense, with low visibility from the thick black smoke, he said.

"Being a restaurant, you have all the tables and chairs to contend with, so it becomes a bit like a rabbit warren in there."

Papesch said the roof was still in place but likely to collapse under the weight of an air-conditioning unit.

"That's the biggest hazard at the moment."

Papesch was unable to say where the fire had started, but when fire crews arrived, it was burning strongly in the kitchen.

Neighbouring Azzurra Fashions and The Christmas Heirloom Company suffered smoke and water damage. It was understood Valentines was also affected.

Hamilton City Council said the footpath outside the restaurant was closed to the public and would remain that way for "some time".