A charity worker told a wealthy woman he was a cancer-cure guinea pig in a bid to scam millions of dollars from her.

Paul Tuba Mika said he had lost everything. His wife and son had died, so he was donating his body to science in an experimental cancer treatment.

Mika told the woman he needed money to pay for airfares to Australia and medical bills - and his 78-year-old target gave him $2.4 million over seven years.

His doctor would even call her to give progress reports.

But there was no cancer treatment and no doctor.

Mika has pleaded guilty to eight fraud charges, which each carry a maximum penalty of seven years in prison. The 55-year-old will be sentenced in the Auckland District Court next month.

His victim - who has name suppression to protect her identity - lives in the upmarket Auckland suburb of Herne Bay. She could not be reached for comment.

She is the matriarch of a family which made its fortune in a retail chain.

Mika met her in 2003 when he was collecting donations for recognised charities.

But in May that year, he asked for money for himself.

He said his wife and son had died, and all he had left was his body, so he planned to participate in experimental cancer testing in Australia.

But he needed money for the costs of the cancer testing, including flights to Australia and medication.

Moved by his plea, the woman wrote a cash cheque for $13,200.

But Mika did not go to Australia and did not take part in any cancer treatment testing.

Instead, he banked the cheque at the Westpac branch in Ponsonby.

For the next seven years, he kept up the charade and continued asking the woman for money.

At times, he would act as though he was extremely ill, and several times the woman received phone calls from a man purporting to be Mika's doctor.

The "doctor" told her Mika was "putting himself through hell" but the cancer testing results were "tremendous".

She gave him 166 cheques between May 2003 and March last year - a total of $2,421,835.

When the fraud was discovered last year, she told police she had no reason not to believe Mika.

Detective Senior Sergeant Hywel Jones, head of the financial crime squad in Auckland, could not comment on the case as Mika had yet to be sentenced.

But he said the fraud served as a warning to others.

"If you're asked to donate to a charitable cause, make sure you check for genuine documentation to prove the collectors are registered," he said.

"Don't take people at face value. And if something does go wrong, the police do take it seriously."