The owner of the venue, Robin Pierson, said Ardern and Gayford booked the venue two years ago but talks broke down after a stoush over catering by a Kiwi celebrity chef, Peter Gordon.
Low-tax lobby group, the Taxpayer's Union released a statement on Sunday saying that "the Prime Minister is breaking the law in using her taxpayer-funded electorate secretary as a wedding planner".
Union spokesman Jordan Williams said, "no one would object to officials ensuring appropriate security at the Prime Minister's wedding – but having an official involved in the venue and catering is totally unjustified."
Williams said the Union would be writing to the Speaker, Trevor Mallard and the Auditor-General to ask them to investigate.
However, a spokesman for the Prime Minister denied there was any wrongdoing, saying Ward, whose partner is Cabinet minister David Parker, was assisting Ardern and Gayford in her capacity as a friend.
"Barbara Ward is a close friend of the Prime Minister and Clarke Gayford.
"She is helping with their wedding in a personal capacity. Her costs are being met privately and the time involved is either outside of work hours or annual leave," the spokesman said.