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Promoters of a machine said to use radio waves to fight disease are set to get a grilling as New Zealand's sceptics gather today for their national conference.

Some users have raved about the benefits of the Rife machine, claimed to use audio frequencies to kill harmful micro-organisms in the body with few or no side-effects.

Yet critics say those offering the machines are taking advantage of vulnerable people with nowhere else to turn.

The Rife machine has been promoted in publications saying it targets 870 conditions from asthma to cancer.

Convener of the three-day 2007 Skeptics Conference in Christchurch, Vicki Hyde, said people who turned to alternative medicines like the Rife machine were often suffering terminal illness or in chronic pain.

"It's very rare that this industry is called to account. There's very little in the way of registration or professional overview," Ms Hyde said.

"If these things could actually demonstrate working beyond the standard placebo effect, they would have spread like wildfire."

At the first day of the Skeptics Conference today, the 100 or so attendees will be asked to put together a box of electronic "bits" to mimic such machines.

"As with many such products in the alternative medicine industry, your box need not actually do anything to win a prize," the conference notes say.

Owner of the Rife Centre in Christchurch, Shirley Montgomery, said she hoped the sceptics would not make judgments born out of ignorance.

"How many of them have tried it, and read up about it?"

The Rife machines had been in use in New Zealand since the early 90s, and were based on science.

There are now three Rife clinics operating in New Zealand.

"If there wasn't anything in these things, they would die a natural death and fizzle out. We have a lot of very happy customers."

Ms Montgomery said she was very ill with chronic fatigue and jaundice when she was introduced to the machines seven years ago.

"I think I owe my life to Rife treatment. I couldn't work or do anything before that."

Users pay $35 per session on the machine at the centre.

Each session lasts about one and a half hours, and the number of sessions a user might need to see a benefit would vary.

Developed by Dr Royal Rife in the United States in the 1930s, according to the Rife Centre website, the machines have even been used by All Blacks to get back on the field faster after injuries.

A similar machine was used to try to treat five-year-old cancer sufferer Liam Williams-Holloway, who died in 2000 after his parents shunned conventional treatments.

Ms Hyde said the public did not hear a lot of complaints about therapies like the Rife machine "because there is nowhere to go to make complaints".