The government does not know how much an MMP referendum might cost, and National's leader is refusing to rule out a question on abolishing the Māori seats.

The ACT Party proposed such a question for the last referendum on MMP in 2011 at select committee, but it was prevented by National's confidence and supply agreement with the then-Māori Party.

After Prime Minister Christopher Luxon's on-air musings about MMP early last month became an election promise to hold a referendum on the electoral system, surprising his National Party caucus, RNZ sought further details about how it would work and what it might cost.

A spokesperson for the Electoral Commission, which would likely run the referendum, said: "We can't provide a ballpark cost for a referendum as it depends on costs at the time".

The Commission said factors affecting potential costs would include whether a referendum was being held alongside a general election, as a standalone poll or by postal vote.

They pointed to about $25 million having been set aside by the Ministry of Justice for the proposed referendum for a four-year term, which the coalition toyed with but ultimately abandoned work on ahead of the election.

A Cabinet paper put the costs of the four-year-term referendum at more like $27m, including $9.21m for a public information campaign.

"The budget would have included operational costs for conduct of the referendum by the Electoral Commission and a public information campaign about the subject of the referendum by the Ministry of Justice," the Commission said.

The Ministry similarly was "not able to supply a 'ballpark' figure for the cost of running various referenda", and also highlighted the estimated cost of the four-year-term poll which would have run alongside the 2026 election.

That breaks down to $18.7m for the Commission to run the referendum, and $6.2m for an accompanying information campaign.

"We do not have cost estimates/historic costs for holding an in-person voting referendum outside of a general election," the ministry said.

When Luxon initially committed to the referendum on Newstalk ZB, he had been asked if he would "do that if you were Prime Minister next year".

RNZ questioned him about whether that meant the referendum would be held in 2027 or if he was Prime Minister next year, and whether - given ACT's previous stance on the referendum - he could rule out a question on the Māori seats.

"Sorry, look, again, you'll see all our constitutional policy as part of a justice policy that will get released closer to the election," he said.

The Prime Minister's Office has ignored multiple requests from RNZ over the past month seeking clarity on those matters.

Parliament's library confirmed the last referenda held in New Zealand during an election were in 2020 on the End of Life Choice Act and Cannabis Legalisation, at a combined cost of $12.9m - some $5.5m under budget. The separate cost of running the election that year was $99.9m.

The Justice Ministry confirmed the last referendums held outside an election were on changing the flag, in 2015 and 2016.

With a budget of more than $15.6m, the Commission delivered the referendums for under $13.6m saying the underspend was "largely due to final voter turnout and related savings in costs of postage and vote processing".

The general election will be held on November 7.