SkyCity has decided to evacuate its entire Auckland precinct, because of smoke from the fire.
That means people will need to leave the Sky Tower, casino, all SkyCity hotels and restaurants and corporate offices.
The company said police were onsite to help with evacuations.
People who were intending to visit SkyCity venues this evening - including restaurants on Federal Street - should make alternative arrangements.
Fire and Emergency said the fire started on the seventh floor - the roof - of the under-construction SkyCity convention centre, before spreading to the sixth.
Bitumen and insulation is on fire and there are also concerns about gas cylinders in the area.
No injuries have been reported. One person is unaccounted for, but Fire and Emergency said they were not concerned about it.
The cause of the fire is unknown.
The Fire Service has sounded the sixth alarm, which means all available resources from Auckland have been called in, with further resources from Hamilton on the way.
Fire and Emergency spokesperson Dave Woon said it was "a very, very difficult fire to extinguish".
Police say there is heavy traffic congestion in central Auckland because of the fire.
There are a number of cordons still in place around Hobson Street.
Drivers are being asked to delay travel into the CBD and the public are being urged to stay away from the area because of the smoke.
Police and ambulance staff are on the scene to assist fire crews and hundreds of people, including construction workers, have been evacuated.
A witness says the flames appear to be spreading.
People have taken to social media to report on the fire.
SkyCity's $700 million International Convention Centre and hotel project which has been plagued by problems, including cost over-runs and construction delays.
The complex is being built by Fletcher Construction and SkyCity.