Parliament's speaker is being slammed by the National Party over an alleged lack of transparency.

Trevor Mallard won't divulge how much his legal fees cost taxpayers after wrongly calling a staff member a rapist.

But just last year, Mallard claimed non-disclosure agreements shouldn't be used when taxpayer funds are involved.

Shadow leader of the house Chris Bishop told Heather du Plessis-Allan Mallard's a hypocrite.

"Taxpayers are entitled to know how much money firstly was spent on the case, but also secondly, if there was a settlement, how much was the settlement for? Because it could be quite substantial." 

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• The Huddle: Does Trevor Mallard need to stand down as Speaker?