The Australian Tax Office has spent more than $80,000 in its latest legal fight against the self-proclaimed royal family of an invented principality in Western Australia's wheatfields.

Last month the founders of the Hutt River Province, self-proclaimed former sovereign Prince Leonard Casley and his son Arthur, lost legal action against the Australian Taxation Office over the payment of eight years' of income tax, worth more than $3 million.

Prince Leonard Casley, the former ruler of Hutt River Province.

ATO legal cost summaries, released under freedom of information laws, show two cases against the 91-year-old Prince Leonard, who abdicated from the throne he established for himself nearly 50 years ago, and his son, Arthur Wayne Casley, have cost taxpayers $81,865.91 to date.

The bill for representation by the Australian Government Solicitor and additional legal counsel topped $40,000 in the case, which focused on back taxes from June 2006 to 2013.