* Fire at under-construction convention centre in Auckland
* Owner SkyCity says no one was injured
* Analysts say $452 mln project biggest underway in New Zealand
* Hundreds of people evacuated from area, traffic gridlocked (Releads, adds colour, details)
WELLINGTON, Oct 22 (Reuters) - A massive fire at an under-construction convention centre in Auckland on Tuesday caused no casualties, according to emergency services, but the smoke billowing across New Zealand's largest city prompted authorities to issue a health warning.
The alarm was first raised at around 1 pm, and hours later, with night falling, firefighters were still battling to put out the blaze which began on the roof of the SkyCity Convention Centre, a building large enough to cover four rugby fields.
Owned by New Zealand casino operator SkyCity Entertainment Group, the construction project, valued at NZ$703-million ($452-million) was reckoned by media and analysts to be the biggest currently underway in New Zealand.
"There have been no reported injuries," SkyCity said on Twitter, adding that everyone was evacuated from the centre.
The nearby Sky Tower, New Zealand's tallest building, a tourist attraction that accommodates hotels and offices, and a casino were also shut until further notice, the company said.
Around 23 fire trucks had joined the operation and fire services expected to work through the night. Earlier in the day firefighters were seen on the roof of the centre desperately trying to extinguish the flames. Witnesses posted videos of the blaze on Twitter.
"It's a very, very difficult fire to extinguish," Dave Woon, spokesman for Fire and Emergency New Zealand, told reporters.
"We are not on top of the fire but with the resources that have just arrived we should be able to make progress. We don't have any injuries at the moment and we don't have anybody missing," he added.
Some media reports said an injured firefighter was admitted to hospital, but Reuters could not verify this independently.
There was no immediate official word on the cause or how many people had been at the centre when the fire broke out.
Traffic was gridlocked in the central business district (CBD) as the emergency services flocked to the site, roads were blocked off, and people were advised to stay away.
Media reports said hundreds of hotel guests in the area were stranded. Forced to find alternative accommodation for the night, and unable to call a taxi, people lugged their suitcases through the streets to leave the affected area.
"If you live or work in the Auckland CBD or surrounding areas, take precautionary measures to prevent breathing in smoke," Auckland Regional Public Health Service and emergency services said in a alert, advising people how to cope.
New Zealand's largest construction firm, Fletcher Building Ltd, is building the centre, but the project has been plagued by delays.
Shares of SkyCity fell 4% to NZ$3.82 on the New Zealand stock exchange. ($1=1.5569 New Zealand dollars) (Reporting by Praveen Menon; Editing by Richard Pullin, Clarence Fernandez & Simon Cameron-Moore)