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A fire which engulfed much of an old theatre in Auckland's Queen St early this morning is being treated as suspicious.

Up to 16 appliances and about 75 Fire Service staff attended the two-floor fire at the old Odeon Theatre, next to the St James, which was so intense that visors on firefighters' helmets began to melt.

Auckland Central senior station officer Chris Scott said the former cinema was not used much these days apart from occasional dance parties and no events were staged there when the fire began.

"We can't think why it would have started. There was no reason why it should have started by itself," he said.

"I know there has been problems there in recent days with vagrants and the investigators will be looking at that and a number of other factors."

Fire Service staff were first notified about the blaze about 3.45am. Mr Scott said it didn't appear significant at first but once firefighters worked their way to the backstage area it became obvious reinforcements were needed.

"We had real issues with heat there and the visors on the helmets were beginning to melt," Mr Scott said.

"We were lucky we have these new hoods that we can use which keeps the temperatures down on the bodies. A year ago this would have been much more difficult to manage.

"Even with all that, it was five minutes away from engulfing the rest of the building."

The fire was largely under control by 7am and remaining crews were looking after hot spots. Fire safety officers were investigating the cause.

Theatre manager John Griffiths said it was business as usual, and once the insurance assessors came in on Monday, repairs would begin.

Mr Griffiths said firefighters did a great job in containing the blaze and everyone was relieved the main St James Theatre was not damaged at all.