Battling severe depression in the harsh glare of public life, Mr Corbell was forced to walk away from his job for four months - a political eternity even in the relatively slow pace of the ACT Legislative Assembly.

The decision, in another parliament, another time, could have spelt the end of a career.

"I could have very easily left the assembly at that time," Mr Corbell said.

"I think if it had been a different Labor leader, a different chief minister, they would have said to me 'you need to step down'."

"Jon Stanhope, to his credit, never required me to stand down. He viewed it for what it was, a period of sick leave."

Mr Corbell would go on to last another decade - 19 years, seven months, and three days in total - making him by far the longest-serving ACT politician.

It is a record unlikely to be surpassed any time soon.

Mr Corbell gave his valedictory speech to a packed public gallery in the Legislative Assembly on Thursday.

Chief among the achievements Mr Corbell counts as his most enduring is putting Canberra on track for 100 per cent renewable energy by 2020. He described it in his speech as the "greatest privilege of my political life".

There is speculation that a post-political career in renewable energy beckons following October's election - a campaign in which Mr Corbell will play no public role.

Mr Corbell said if an opportunity presented itself in the renewable energy sector, he would "certainly grasp it and seize it".

"For me, the pursuit of renewable energy, of climate change and sustainability, is not an end of itself," Mr Corbell said.

"It's an end towards a more just and equitable society ... that we can enjoy, and future generations can enjoy."

Mr Corbell's valedictory speech championed light rail, the centrality of human rights, of a unified emergency service, and the importance of the ACT's doomed marriage equality law.

His speech was met with a standing ovation, and praise from the other side of the chamber. Opposition Leader Jeremy Hanson spoke of the pair striking up an "unlikely friendship" while exercising at boot camp.

The two would work out side-by-side in the morning, attack each other relentlessly at work, and then have a friendly chat at the end of the day.

The end for Mr Corbell was unceremonious. He was relegated to last on the Labor left faction's pre-selection ticket, a move that ruffled feathers in some sections of the party.

For his part, the deputy chief minister said he held no ill-will over the events that precipitated his retirement, and said he had decided against contesting October's election long before.

"What happened in the party was a catalyst, but not the reason for my decision," he said.

"The fact is that after nearly 20 years of public life as a relatively young man, I'd certainly like to do other things."

Mr Corbell said the months after caretaker would be dedicated to his family, fitness, a newfound, mid-life love of motorbikes, building a sustainable home, and less time worrying about his work.