Chief Minister Katy Gallagher and Opposition Leader Zed Seselja will face off on issues including euthanasia, gay marriage and religion in schools at the Australian Christian Lobby's (ACL) ACT election forum tonight.

But ACT Greens leader Meredith Hunter has been excluded from the event because the lobby group said it was too ''cumbersome'' to host a forum with the three party leaders.

The lobby's ACT branch director Richard Thackway said the Greens had also ''generally avoided in the past getting in front of a Christian constituency and making their points''.

''They have been really reluctant because their views on marriage, euthanasia, abortion, generally don't go down well in the forums we have,'' he said.

Mr Thackway said the ACL traditionally held its major forum with just the Chief Minister and Opposition Leader, but the Greens would be invited to smaller ''meet the candidates'' events to be held in each electorate.

But Ms Hunter said the party had ''never been reluctant'' to attend ACL events in the past, including meet the candidates forums.

''My understanding is the purpose of an election forum is to hear the point of view of all parties and independents, it's not only about inviting the people who you make a judgement that they agree with what your views are,'' she said.

Mr Thackway said tonight's debate would be one of the territory's biggest pre-election forums, with up to 700 Canberra residents expected to attend.

Ms Gallagher and Mr Seselja will address the crowd on key election issues and that will be followed by questions from some of the territory's religious leaders.

At the ACL's 2008 election forum, Attorney-General Simon Corbell was heckled and booed by audience members over his stance on gay marriage.

Mr Corbell's bill restoring civil unions for same-sex couples successfully passed the Legislative Assembly last week, and the Labor MLA is working with the Tasmanian government to develop full same-sex marriage legislation.

Ms Gallagher said she expected tonight's forum would also include discussion of religion in schools and euthanasia. ''I don't suspect everyone tomorrow will like every answer I give,'' she said. ''It's not unusual for politicians to put themselves in that position.''

Mr Seselja said he would focus on the health system and cost-of-living pressures. ''As with all community forums, I'll be discussing the Canberra Liberals' plan for better basic services with lower rates and charges,'' he said.