And the Concerned Citizens of Canberra group - which had just six members and $2159 in assets in October last year - has been told it must stump up $25,000 as security before the case can go ahead, after fears were raised about its ability to pay court costs if ordered to do so.

The application to build the 500-capacity mosque on The Valley Avenue was approved by the city's planning authority in August. That decision was made in the face of strong opposition by the Concerned Citizens of Canberra group, which tried to drum up opposition to the mosque, delivering thousands of flyers to nearby homes.

The group then launched legal action against the ACT Planning and Land Authority's decision to approve the work.

Initially, court proceedings were focused on a decision by the planning authority to not extend the period of public consultation on the proposed mosque.

The Concerned Citizens of Canberra then extended its action to challenge the decision itself.

But the group missed a deadline to make that challenge after it mistakenly went to the ACT Civil and Administrative Tribunal instead of the ACT Supreme Court.

It claimed it was misled over which forum was appropriate to mount the challenge, but that ''unimpressive'' explanation was not accepted by Acting Supreme Court Justice Margaret Sidis.

In a judgment on Tuesday, Justice Sidis gave the group the chance to amend its application against the mosque's approval.

But the court ordered the group to pay a $25,000 security by late April, after evidence suggested it would not have enough money to pay costs if the court ordered it to do so.

Justice Sidis noted there was evidence ''individuals'' standing behind the group had the means to pay the $25,000. She said one of those individuals was Irwin Ross, a Christian fundamentalist activist who lived at least 15 kilometres from the proposed mosque site and acted as the Concerned Citizens of Canberra spokesman.

Mr Ross had attempted to join as a party to the proceedings, while keeping his name suppressed to protect himself from ''further unfavourable media attention''.

But Justice Sidis dismissed his attempts to join the legal battle, saying she was not satisfied the interests of justice demanded it. She said she was ''especially concerned'' at Mr Ross' attempts to have his name and other details suppressed.

The group argued it should not be made to pay the security, at first saying its ''genuine concern'' at the decision-making process should not be frustrated by a lack of resources. It then said that the public interest issues in the case allowed the court to exercise its discretion to not force the group to pay the $25,000 security.

It listed two issues of ''public interest'' arising out of its opposition to the development: protection of the environment, and the ''purposes of maintaining religious or ethical standards''. Justice Sidis rejected both arguments.

The group must now submit an amended challenge to the court and pay the security by April 30.

If the $25,000 is not paid, the court will consider an application by the Chief Planning Executive to throw the case out.

The Canberra Muslim Community organisation has chosen not to be a party to the proceedings but is closely observing the legal battle.