Rotorua's renowned Polynesian Spa is to be prosecuted by Occupational Safety and Health over the death of a Wanganui artist who collapsed in a thermal pool in May.

Joanna Margaret Paul was taken off life-support two days after she was found by another swimmer floating face down in the hot mineral Priest's pool.

A coroner's inquest has not yet been held but it is thought that she may have collapsed from hydrogen sulphide fumes before drowning.

The gas, which gives Rotorua its rotten eggs smell, has been blamed for 11 deaths in the city since 1946, including that of Austrian actress Ellen Umlauf-Rueprecht, who died in her motel room in February 2000.

The Polynesian Spa closed eight outdoor pools that are fed from the Priest and adjoining Radium springs in August when tests ordered by OSH after Ms Paul's death found that gas measurements were higher than the accepted exposure standard.

The other 25 geothermal pools at the complex have a separate source of deep spring alkaline water and have remained open to the public.

It is understood that the spa company will now be prosecuted over the way it handled the accident, but OSH's regional manager, Murray Thompson, has refused to elaborate on what the exact charges will be.

The spa's management has also refused to comment but it is understood it has hired a Queen's Counsel to defend the charges and represent it at Ms Paul's inquest, which will be held next year after OSH's prosecution is resolved.

Ms Paul's family have also been carrying out their own investigations into the 57-year-old's death and examining previous cases of hydrogen sulphide poisoning, including that of Ms Umlauf-Rueprecht, who spent some time bathing in thermal pools before she died.

Sister Mary Paul said the family have asked that further testing be done on the pools before the inquest.

"We understand that not enough is known about the effects of hydrogen sulphide and we're concerned that proper testing and monitoring needs to be carried out," she said.

A Canterbury University study last month suggested Rotorua's sulphur fumes could also cause chronic health problems.

It found the incidence of respiratory diseases was five to 10 times higher than normal in parts of the city close to geothermal activity.