Former Speaker, and soon-to-be ambassador to Ireland, Trevor Mallard's decision to issue Winston Peters with a trespass notice from Parliament cost taxpayers' more than $23,000 in legal fees.

Answers to written Parliamentary questions show the fee included charges for reviewing and preparing documents as well as various attendances and correspondence.

Mallard refused to comment on the bill when approached.

But National's shadow leader of the House Chris Bishop says this is Mallard's "final insult to taxpayers'".

"He's cost the taxpayer another $23,500 in legal bills because of his illegal attempts to trespass a former Deputy Prime Minister and a bunch of other MPs from the Parliamentary forecourt, after the protests earlier this year."

Along with Peters, Mallard issued the trespass notices to former Act leader Rodney Hide, former Māori Party co-leader Marama Fox, former NZ First list MP Darroch Ball and former National MP – turned Democracy NZ leader – Matt King in early May.

But he quickly withdrew the orders, saying in a statement at the time: "The advice I have received is that it is no longer necessary to retain trespass notices for these five people".

Despite this, Peters' lawyers contacted the Speakers' office saying legal action would be pursued, if Mallard couldn't provide: "Specific explanations, documents, and referenced authority with which he, in a free and democratic society, trespassed hundreds of New Zealand citizens from the Nation's Parliament".

More than two months later – a day after Mallard vacated the Speakers' chair – the Speakers' Office issued a public apology to Peters.

The statement said the trespass notice and warning was "unreasonable and irrational" and was an "unjustified limitation on Mr Peters' right to freedom of movement".

The media release noted that this statement was issued while Mallard was Speaker.

But it was publically released by new Speaker Adrian Rurawhe – it was his second day in the job.  

Bishop said the fact the Speakers office was forced to spend more than $23,000 on legal fees shows it's clear Mallard overreached.

"Trevor Mallard is a very experienced Parliamentarian; he's now been forced to apologise to Winston Peters; he's accepted that he's acted illegally and irrationally and the proof is in the pudding."

He notes this bill is on top of the $330,000 in legal fees taxpayers had to cough up after Mallard falsely accused a Parliamentary staff member of rape.  

In June, Prime Minister Jacinda Ardern revealed Mallard would be stepping down as Speaker in August.

She later revealed he had been appointed as New Zealand's ambassador to Ireland.

He takes over early next year.