Opposition leader Zed Seselja.

Zed Seselja. Photo: Jeffrey Chan

ACT Liberal senator Gary Humphries has challenged his internal rival, Zed Seselja, to a debate before the two men face off in their preselection showdown on Saturday.

But Mr Seselja has refused to debate the senator unless he promises ''no more public destructive commentary about the party''.

Both men agree the debate should take place away from media scrutiny, with just party members present.

Senator Gary Humphries.

Senator Gary Humphries. Photo: Andrew Meares

The senator suggested the debate should be about the big issues likely to feature in the federal election campaign, such as the public sector job cuts, the economy and action on climate change. He suggested the event be held this week and be moderated by a party member acceptable to both candidates. Senator Humphries and Mr Seselja would make a speech to party members, who would have the opportunity to ask questions.

But the request from Senator Humphries began a heated email exchange between the candidates.

''As you know, I am concerned at the exclusion of so many ACT party members from this preselection process, hence my proposal that all of them be invited to attend this debate,'' Senator Humphries wrote.

But Mr Seselja said he would not debate until ''important issues'' were resolved, including that the debate not become a forum for discussion of the preselection process.

''I also ask that you desist from making public comments which bring the party into disrepute,'' he wrote. ''I'm concerned about the comments of some of those aligned with you such as [former party president] Gary Kent, as well as your comments claiming the process is unfair, your incorrect claims that you didn't know about the preselection process until January and your incorrect and offensive claims of a deal in relation to the leadership of the parliamentary Liberal Party.''

The former leader said both candidates should publicly affirm the independence of the party's internal electoral returning officer and commit to abiding by the decision of preselectors on Saturday.

But Senator Humphries said it was unreasonable to link his views about the preselection to the format of the debate. ''It is fundamentally wrong to exclude over 400 ACT Liberal Party members from the right to vote in possibly the most important decision the division will take this decade,'' he wrote.

''My concerns had been deepened by the discovery in the last few days that a further 15 or so members who were previously qualified to vote have been disqualified because of technical irregularities.''.