A large fire that caused millions of dollars in damage to a commercial Christchurch building is not deemed suspicious.

Specialist fire investigator Bruce Irvine said the Friday night blaze "gutted" about 30 per cent of the Wairakei Rd, Burnside building, which is owned by Tait Communications.

Irvine used a drone to view the damage on Saturday morning. He would resume the scene examination with insurance investigators on Monday.

DAVID WALKER/FAIRFAX NZ The aftermath of the Friday night fire at Tait Communications' Burnside building.

"At this stage the cause is still undetermined, but we're not treating it as suspicious. It is a fairly sizeable and complex fire," he said.

Two cleaners raised the alarm about 6pm.

One of them told ONENews: "We were in cleaning and the light blew in one of the working areas in the ceiling and it just caught fire. So I rang 111 and this is the result."

DAVID WALKER/FAIRFAX NZ Most of the damage was to the half of the building that was empty or leased as office space.

It took firefighters about two hours to get the blaze under control. Forty-two personnel were there at the height of the blaze.

Crews stayed on site overnight, but there were not flare-ups, Irvine said.

The severe damage was to parts of the building that were either empty or office space leased to two tenants, MimoMax, a subsidiary of Tait Communications, and the New Zealand Manufacturers Employers Association.

GEORGE HEARD/FAIRFAX NZ Forty-two Fire Service personnel were at the scene at the height of the blaze.

An area used to store Tait Communications electronic equipment was relatively unscathed thanks to firefighters efforts, Irvine said.

"We've managed to stop the loss of critical equipment."

The badly burnt areas were "totally destroyed", Irvine said.

ALDEN WILLIAMS/FAIRFAX NZ Smoke coming from the scene.

The building is one of three on the site owned by Tait Communications.

Company spokesman Bryn Somerville said on Saturday it was too early to know if demolition would be necessary.

"We're going to have to see what they [the Fire Service] say."

ALDEN WILLIAMS/FAIRFAX NZ Firefighters battle the blaze.

DAMAGE WILL COST MILLIONS

Tait Communications chief executive Garry Diack said on Friday night that half of the building, including the leased office space, was completely "burnt out".

He told RNZ it had been undergoing refurbishment for a possible sale and would cost millions to rebuild.

Electronic materials stored in the building had minor damage to packaging, but the business would not be affected.

"No-one was harmed, which is good, and the impact to the business is minimal, but it's disappointing we have an asset that has been damaged."

Christchurch-based Fire Region Commander Steve Turek said the fire was well developed when firefighters arrived.

Firefighters cut their way through the front roller doors and attacked the fire from the back and above.

Crews, wearing breathing equipment, worked "quite arduously in heavy smoke conditions", he said.

* An earlier version of this story said staff members were unaccounted for.