She will be co-convenor of a new parliamentary group, with federal Treasurer Joe Hockey, as part of an ambitious 10-year plan for constitutional change.

The group was announced at the National Press Club on Wednesday by Peter FitzSimons, chairman of the Australian Republican Movement and a Fairfax Media columnist.

The republican movement wants a national plebiscite on an Australian head of state by 2020, followed by a referendum proposing a specific republican model by 2025.

Should a plebiscite on a republic be held within that time frame, it could be the fifth time in as many years Australians were asked to go to the polls. Votes on marriage equality and constitutional recognition for Indigenous Australians, as well as two federal elections, are also expected.

In 1999, a referendum on a move to a republic was defeated 55 per cent to 45 per cent. The ACT was the only jurisdiction to vote in favour of a republic.

Senator Gallagher, a former ACT chief minister, said there was acceptance of the lessons learnt from 1999, such as about divisions in the "yes" campaign.

"I think there's also understanding now there needs to be stages about the way you progress to a referendum," she said.

"It's a long-term discussion for a long-term change.

"Politicians have a role to play but, ultimately, this is something the Australian community has to get behind; it has to be a much broader discussion.

"If we are looking at a five- to 10-year campaign, hopefully it is beyond the reign of Tony Abbott."

Mr FitzSimons said support for an Australian republic was strengthening again, with 47 per cent of voters polled by Essential Media Research in favour of replacing the British monarch with an Australian head of state.

"It's time for us to be entirely self-governing," he said.

"We propose it starts with a simple question to be put before the Australian people some time in the next five years: do you support replacing the British monarchy with an Australian citizen as the Australian head of state?" he said.

"Bingo, simple as that. We reckon the 'yes' vote will look like Phar Lap at Flemington, like Bradman at Lord's – well ahead of the field and looking good."

Mr FitzSimons said it was essential republicans came together and agreed on a model before a referendum, rather than face a repeat of the 1999 vote, when arguments between republicans about different models contributed to the defeat of the "yes" case.

He said he favoured a minimalist model, with the head of state, who would still be called the governor-general, chosen by a two-thirds majority of Parliament.

"It is the most likely to succeed, as it addresses the foremost concern of the if it ain't broke, don't fix it crowd. Essentially, we are not fixing it. We would be snipping one unsightly apron string," Mr FitzSimons said.

The Australian flag would not be part of the debate.

Mr FitzSimons warned Australia would only make the change if Mr Abbott, a strong supporter of the constitutional monarchy, was no longer Prime Minister: "The reality of this is we won't get this over the line without bipartisan support."

A spokesman for Mr Hockey said he "has long advocated his views on this issue".

"They are a matter of public record and those views haven't changed."

Earlier this year, Opposition Leader Bill Shorten recommitted Labor to pursuing an Australian republic.