Auckland's landmark Tepid Baths building is to close for four years to correct structural damage. The closure will mean the loss of up to 50 jobs.

The 95-year-old building at the Viaduct will undergo a $12 million renovation. It has been open since 1914, with facilities including a 25m lap pool, a fitness centre, aerobics studio and a children's pool.

Up to 50 YMCA staff employed at the baths will have no job come May 17 next year, when building work begins.

Auckland City councillor Greg Moyle, chairman of the arts, culture and recreation committee, said the decay of the building meant the council had no choice but to shut it.

"It is with regret that we will have to enforce a temporary but long-term closure of this much-loved facility," Mr Moyle said.

"However, in the interests public safety, an interim closure of the Tepid Baths is the only feasible option while we fix the structural issues."

The council had made plans for a $3 million renovation in time for the Rugby World Cup in 2011. But structural reports - indicating there was severe damage - forced the council to change its plans and put $12 million towards a large-scale renovation of the heritage building.

Mr Moyle said design and consent processes would take about two years to complete.

"And then we've got to build the thing," he said. "[The pool] is 95 years old. When it was built, they probably never thought it would last 95 years. It's been patched up repeatedly in the last 10 years but no one's been tasked to take on the problem properly. Now we are."

Around 195,000 people use the baths facilities each year, an Auckland City council spokesman said, with 80,000 of those people users of the fitness centre.

YMCA Auckland chief executive John Fairhurst said: "We're devastated for our staff and for our members. This is very much a family and we all care for each other."

Mr Fairhurst said that with around 800 staff members working at various sites, it would be hard to relocate staff working at the baths.

"We're looking to [relocate] staff, but sadly there will be a lot of people without jobs," he said.

Engineering reports show the building will be safe for the next six to nine months, however a three-month monitoring period is under way.

Although reconstruction of the building is due for completion in 2014, in time for the building's 100th birthday, Mr Moyle said it was possible that the work would be finished earlier.

"Is it going to be exactly the same layout as it is now? I don't know. It's an opportunity to take a step back ... and make it better so that it can be used for another 100 years."

The council is talking to private pool operators so that people will still have access to swimming facilities.