ACT leader Don Brash says the party is yet to decide which voting system to support in November's electoral referendum.
A lobby group supporting retention of MMP says it expects a new anti-MMP campaign will soon be launched.
But Dr Brash says a report linking ACT to such a campaign is unfounded.
While he personally favours Supplementary Member Voting, Dr Brash says ACT is yet to formally decide whether it will support one of the five voting systems on offer at the referendum.
Campaign for MMP spokesperson Sandra Grey says a new lobby group opposing MMP will soon be launched, likely with the backing of big business.
A publicity campaign by the Electoral Commission is to begin this week about the upcoming referendum, which will be held on election day, 26 November.
Voters will have five options to choose from including MMP, Supplementary Member and First Past the Post.