Ms Porter, who would be 74 by the next election in a year's time, said age had never been a barrier to her political career but poor health was forcing her out – not, she said, any pressure from the Labor Party. Colleagues had respected her decision but still tried to convince her to stay.

"I think I could easily have won another seat and there is a lot of regret for me in this decision but it's become evident to me I can't do my job properly because of my health," she said.

The British-born former nurse, who went on to help establish what became Communities at Work in Tuggeranong, entered the assembly in 2004 and by the 2012 election had the highest number of first-preference Labor votes of anyone bar the popular then chief minister, Katy Gallagher.

Speaking exclusively to The Canberra Times on Thursday, Ms Porter said Chief Minister Andrew Barr had asked her to stay until the next election in October 2016, but she might have to leave earlier due to her chronic arthritis which was affecting her ability to "give 100 per cent". It had been " a great honour" to serve in the assembly.

"I'm really grateful to the constituents for giving me this opportunity to be here. I really think it's been a really wonderful opportunity. I've really enjoyed the constituent work and also the committee work, which I think is really important," she said.

If Ms Porter did leave earlier, that would mean a countback for her seat, with remaining Labor candidates for Ginninderra from 2012, Jayson Hinder and Glen McCrea, in line for the position – and then facing an election in less than a year,

A former chief executive officer of Volunteering ACT, Ms Porter plans to continue to work with community groups, focus on her family including husband Ian De Landelles, five children, 11 grandchildren and one great-grandchild, and to follow her champion racehorse, Fell Swoop, which she co-owns.

She said her husband had "been her rock" and her family had always supported her in her career. She also paid tribute to her staff.

Mr Barr said on Thursday that Ms Porter could have definitely contested another term but he respected her decision to retire.

"She would have been one of the first candidates pre-selected and her personal vote at the last territory election and, indeed,over the course of her time in the assembly has been amongst the strongest," Mr Barr said.

"If you take out the chief ministerial vote, she would have been one of the top vote-getters, so that's testimony to her grass-roots campaigning style. I hope she can continue through this parliamentary term but we completely understand that determining factor will be her health and her personal commitment to give 100 per cent.

"Mary has always been someone who has given her all for her constituents, for the Labor Party and for Canberra."

Ms Porter said she had always wanted to be an effective local member rather than pursue higher positions, meeting her constituents via mobile offices at shopping centres. She recently had to suspend the mobile offices due to her health.

"People didn't vote Labor or Liberal, they just voted for Mary," she said.

"I've been able to deal with thousands and thousands of constituency matters. And, for me, it's a great satisfaction because I know I have done that job well. I've now started to realise I can't do that running on empty. And I'm now starting to run on empty because of the pain."

Ms Porter said she would leave the assembly after close to 12 years with a great amount of pride, her biggest gains being in securing improved security for people living in retirement villages, in pushing for restorative justice and in raising the profile of end-of-life issues.

She would not say definitely if she supported voluntary euthanasia but her big regret was that there was still a barrier at a federal level, in the form of the Andrews Bill, to the ACT being able to legislate for it.

"We're still at that stage where the public can't even make a decision that, for me, is the hard part," she said.

Both Ms Porter and Mr Barr acknowledged Labor would find it more difficult in Ginninderra without her personal vote but there was also a chance for renewal.

"It will certainly make a difference but we've got some very good candidates," she said.

Facing the conclusion of her "third career", Ms Porter said being in her 70s in politics had never been an issue.

"I don't think age should be any barrier to anyone coming into this place. I don't care how old you are or how young you are. But with the young, I would caution, I do think you need some life experience before coming into this place to think through some of the difficult decisions you have to make," she said.

Mr Barr agreed.

"I think she worked harder than opposition members half her age. She's always been one of the hardest-working members of government," he said.

Ms Porter had served under three chief minister – Jon Stanhope, Katy Gallagher and Andrew Barr – and believed Labor could succeed at the next election, even against the divisive issue of light rail, a project she supported.

"None of them have been short-term politicians. They always looked to the future. They always looked to the big picture," she said.

The Liberal Opposition has also paid tribute to Ms Porter, Assembly Speaker Vicki Dunne and Deputy Opposition Leader Alistair Coe thanking her for her service to Canberra.

"Mary has been terrific to work with and I thank her for the work she's done," Mrs Dunne said. "Politics promotes adversity but Mary has been a person to bridge that divide. I wish her well into the future."

Mr Coe said Ms Porter's retirement would be a loss for the Assembly.

"I very much respect the way Mary operates and I've enjoyed spending time with her over the years in the Assembly, on committees, at events and on the hustings," he said. "Mary has a compassionate and professional approach to representing the community and I thank her for her service to Canberra."