Single-bookkeeper accounting practices used by many charities may have left the door open to an alleged $500,000 fraud against the Roman Catholic Archdiocese of St. John's, a forensic accountant says.

Al Rosen made the comment following an announcement by the Archdiocese of St. John's that its former business manager is suspected of taking more than $500,000 after an independent auditor was hired to look at the archdiocese's books.

Archbishop Martin Currie said the church is asking police to investigate its finances. (CBC)

The archdiocese has asked the Royal Newfoundland Constabulary to investigate.

Rosen said charitable institutions often have one person in charge of the books, and their work isn't double checked.

"Unless you hire somebody to do that and not leave it in the hands of the person who is committing these situations, without the outside expertise, you're not going to catch [fraud] very easily," said Rosen.

He said internal fraud happens frequently at charities and churches, but it often flies under the radar.

The archdiocese alleges its own investigation found a former business manager paid the money to himself and his pension account.

Archbishop Martin Currie told CBC News the archdiocese planned to ask police to investigate.

"It's a shock for everybody, because he's been with the archdiocese for roughly 38 years, and everybody had given him their complete trust. He was trusted fully you know, and he's broken that trust. That's what's saddest more than anything," Currie said.

Bill Power, the archdiocese business manager, resigned from the church last fall.

Power, of Outer Cove, near St. John's, would give "no comment" when called by CBC News on Monday morning.

The church says it has brought in tougher financial guidelines since last fall.

The archdiocese covers most of the Avalon Peninsula and also the Burin Peninsula.

Donated money is used to fund chaplaincy services in hospitals such as the Janeway, St. Clare's, the Health Sciences Centre, the Burin Peninsula Health Care Centre and the Miller Centre, and in seniors homes and long-term care facilities.