Mr Jeffery's tenure as a Liberal parliamentarian will be short. If he is elected at Thursday's countback, he will officially take up his seat at midnight on Friday and be in the debating chamber on Monday morning for the final two weeks of sittings for the term.

On September 9, the ACT government goes into caretaker mode before the October 15 election.

Mr Jeffery, who turns 82 in December, will make history as the oldest person to sit in the ACT parliament, and he also looks to be beating federal records, older than anyone to have entered the federal parliament in its more than 100-year history.

He is set to replace Liberal Brendan Smyth, who left politics suddenly two weeks ago, when appointed by Chief Minister Andrew Barr to the position of Commissioner for International Engagement, a five-year job with a base salary of $251,000 a year as part of a $300,000 package.

Rather than holding by-elections when there is a vacancy, the ACT counts back votes from the previous election to elect a replacement.

With Mr Smyth's departure, each of the ballot papers that contributed to his election in the electorate of Brindabella at the 2012 election will be redistributed at the countback on Thursday.

The votes are redistributed to all candidates from 2012 who nominate for the countback. Mr Jeffery is the only Liberal left in the line-up. Three Labor candidates were not elected, and two had put their names into the countback by Wednesday – Karl Maftoum and Rebecca Cody, both of whom are standing again in the October election. It was unclear on Wednesday whether Labor's Mike Kinniburgh would also nominate. Any other unsuccessful candidates in Brindabella are fee to nominate also, but Mr Jeffery is expected to win.

At the point of Mr Smyth's election, he had 17,281 ballot papers which were worth 10,594 votes, the quota for election. Those papers will now be redistributed to candidates preferenced after him. Candidates have until noon on Thursday to nominate and the countback is at 12.30pm.

Mr Jeffery, who runs the Tharwa General Store, said he was "in a learning curve, just taking it one step at a time".

But he was pleased at the opportunity.

"It's something different to put an extra couple of months in towards the end of my lifetime, the end of retirement," he said. "It's a bit hard to explain because it came so out of the blue. All I can do is just take one step at a time for the next couple of months."

His family, too, was right behind him.

"My wife thinks it's a great idea. After all the things I've been through all my life, she thinks it's great to take a another step forward," he said. "I'm going in there with a very positive attitude just to make the most of a couple of months."

But he accepted there was very little he could achieve, given there were only two sitting weeks left. He was also unsure about practicalities such as staff.

"I don't know what the story is, but I presume there'll be staff there," he said. "I'm a bit dark. It's all new to me."