A search is underway to find the owner of an unclaimed fortune left behind by a mystery milkman, who worked for decades in North Perth.

When George Francis Thompson dies in May 1994, his estate was worth about $600,000 but has now grown to $915,703.

The hunt for the milkman's relatives has so far baffled Perth historians who are now asking for the public's help.

Known up and down the streets of North Perth, Mr Thompson went door-to-door delivering milk from the 1950s.

He lived at 147 Raglan Road in the suburb, but his neighbours have all moved or passed away.

Newspaper clippings show Mr Thompson was prosecuted under the Milk Act for diluting his milk with water in 1954.

He was also fined five pounds when his horse and cart collided with a car in 1953. He's buried at Karrakatta Cemetery.

Susanna Luliano, the manager at the City of Vincent Local History Centre, said she was hoping to distribute the estate to a long lost relative - or relatives.

"If they can prove they are a descendant, and I think there are all sorts of different ways that Treasury can have that proven, potentially there's over $900,000 for someone to claim" she said.

"Our guess is that he came from overseas, possibly to the eastern states first, and then moved over from the east to Perth which makes his trail a little bit harder to track," Dr Luliano said.

The milkman's estate is the biggest individual account in a $73 million pool of unclaimed money held by the Western Australian government.

There are more than 500,000 individual pools of cash that can be claimed, regardless of how long the money has been held.

They include betting and lottery accounts, share dividends and other unclaimed credits.

There's so much unclaimed money sitting in the bank that the state's Public Trustee is planning to employ someone to distribute the cash.