An Auckland pensioner stole the identities of two dead children to reap $450,000 in benefits in a case the government says is rare and unrepeatable.

Colin Diedrichs, 82, a retired gardener, claimed extra superannuation payments for more than 20 years using the personal details of two boys who both died decades ago.

He has now admitted 20 criminal charges and will be sentenced in Auckland District Court next month, the New Zealand Herald reports.

"To get more money I suppose," he told the paper when asked why he stole the boys' identities.

He admitted he did not really think of the boys' families.

The paper compared the case with that of shamed former Act MP David Garrett, who, inspired by the novel The Day of the Jackal, successfully applied for a passport using details he found on the cemetery headstone of a two-year-old boy.

Ministry of Social Development fraud head Mike Smith said the case was "rare and cannot happen today".

A person would need to go to extraordinary lengths to repeat the crime, and controls were considerably stronger now than they were 20 years ago, he said.

More than $350,000 worth of Diedrichs' cash and assets had been frozen and was the subject of court action, Mr Smith said.