The ACT government has confirmed those legal costs were likely to total more than $200,000, far exceeding the $35,000 in security the group was forced to pay into the court early on in the piece.

Concerned Citizens lost its fight against the mosque in November 2015, and a costs hearing was heard last February.

In the costs hearing, the group had broadly argued the case was in the public interest, and therefore costs should be awarded either in its favour, or not at all.

It also said a costs order against it would be unfair to its fundamentalist Christian president Irwin Ross, because he would be liable to pay even though he had been denied by the court the opportunity to be a party to the case and have his say during the proceedings.

But on Tuesday, Justice Richard Refshauge of the ACT Supreme Court rejected the group's arguments.

"There was not, apart from mere assertion by the applicant, any rationale or explanation given as to why this was public interest litigation," the judge said.

The judge noted Mr Ross' presidency of the group suggested he had some influence over the proceedings, and that he seemed "intimately involved" in the last application.

"It is simply disingenuous, if not verging on the deceitful, to suggest that the interests of Mr Ross could not be addressed by the applicant in its submissions, at least to a greater extent than it was actually done," Justice Refshauge said.

The 500-capacity mosque at a site on The Valley Avenue in Gungahlin was approved by the ACT planning authority in 2012.

Concerned Citizens launched court action that year against the decision, and against the authority's decision not to extend its consultation period.

It was dealt a major blow when then ACT Supreme Court Master David Mossop found it wasn't directly enough affected to be able to mount the challenge in July 2014.

The group lodged a late appeal against Master Mossop's decision, but Justice Refshauge found they had no reasonable prospects for success.

In dismissing their attempted appeal, Justice Refshauge described the group's interest in the development as no more than that of an "intermeddler or busybody".

Early on, there were concerns about the group's ability to pay costs. Concerned Citizens of Canberra Inc has a small member base, about 18 at the time the case was heard, and not much in cash or assets, the court has previously heard. The court forced the group in 2013 and 2014 to stump up $35,000 in security before the case went ahead.

In February 2015, the government warned it had already incurred $93,143 in court costs and estimated a further $30,050 would be spent before the finalisation of the first hearing before then Master David Mossop.

On Tuesday, Mr Ross told Fairfax Media he had to speak with his lawyers, and would not be commenting beyond confirming he was still president of the group.

In a statement, the government said the territory routinely pursued its costs when it had successfully pursued or defended a matter, other than in exceptional circumstances.