ACT Opposition Leader Zed Seselja.

ACT Opposition Leader Zed Seselja. Photo: Katherine Griffiths

Outgoing ACT Opposition Leader Zed Seselja has broken his silence over claims of process irregularities in his preselection challenge to serving senator Gary Humphries as more prominent Liberal federal MPs line up to support the incumbent.

The local Liberal leader went to ground on Tuesday after questions were raised about the timing of his challenge and whether branches had been in effect ''stacked'' to ensure he won the party vote.

The party's preselection management committee held an emergency meeting to assess the claims but found there had been no wrongdoing.

On Friday, Mr Seselja explained why he had kept quiet since announcing his candidacy for the number one spot on the Liberal's ACT Senate ticket.

''Certainly I believe it's not my role to give a running commentary on what is an internal process,'' he told Fairfax Media.

''Those who have made the claims have had those claims refuted pretty clearly by the people who look after these things in the party's organisation. There seems to be no real response to that. I guess they'll have to explain why their claims don't stand up.''

Senator Humphries has cried foul over the process, saying he was led to believe Mr Seselja would not challenge him.

He said he was only informed otherwise the night before nominations closed, which did not allow time for many party members to qualify to cast votes in the preselection. ''I'm just concerned that the very late announcement of the candidacy has effectively

meant that many people have been caught unawares and can't vote at the preselection,'' he said.

Other prominent ACT Liberals have also complained about the process, with some suggesting ''qualifying meetings'', which allow members to become eligible to vote, had been corrupted.

Meanwhile, more federal Liberal Party frontbenchers have publicly thrown their support behind Senator Humphries.

Shadow treasurer Joe Hockey said the incumbent had his endorsement. ''Gary's an honourable man. He's a formidable advocate for Canberra and always first in the door to act on behalf of his constituents,'' he said. ''Gary's a decent human being who properly reflects the aspirations of everyday Canberrans.''

The Liberal Party's leader in the Senate, Eric Abetz, also came out in support. ''I've known Gary for 35 years. He's been a Senate colleague of mine for 10 years,'' he said. ''As you would expect, I am writing a letter of endorsement and reference for him.''

The two senior Liberals follow federal Opposition Leader Tony Abbott, who offered his support for Senator Humphries immediately after Mr Seselja announced his challenge on Monday.

Mr Seselja said on Friday that he wasn't surprised by Mr Abbott's endorsement for Senator Humphries.

''Certainly I think John Howard took the approach that he would generally back the sitting member,'' he said. ''I'm not sure if Tony Abbott has taken that same approach but that appears to be in the tradition of Liberal leaders that they will generally back the incumbent. I respect that but I wasn't overly surprised.''