Winston Peters' superannuation trial could cost taxpayers $1 million

The High Court battle over the leak of Winston Peters' superannuation overpayments could cost the taxpayer around $1 million.

Peters sued five current and former top bureaucrats over what he says was a politically motivated leak of his pension overpayments to the media just weeks before the 2017 election.

Documents obtained by Newshub under the Official Information Act show the pre-trial cost to taxpayers was at least $537,644. Some pre-trial invoices aren't included in that figure.

Two QCs represented the defendants - former National Ministers Anne Tolley and Paula Bennett, former Ministry of Social Development CEO Brendan Boyle, and State Services Commissioner Peter Hughes - and their trial costs are expected to reach between $200,000 and $300,000.

That means the total cost of the trial to the taxpayer, once all invoices are in, could be about $1 million.

And if  Peters wins, he could be awarded costs and damages, making the final cost even higher.

"I would expect a costs award of $80,000 to $100,000 to Mr Peters if he is successful," says FortyEight Shortland barrister Tim Rainey.

Peters argued his superannuation information was private information that should never have been disclosed to Tolley and Bennett.

The Ministers argued they're not responsible for the leak and they needed to be told about Peters' overpayments to ensure he was treated fairly.

The final cost of the trial will be known early next year, around the same time the judge releases his decision.