'I was very difficult to be around': Gallagher's pain and grief

Speaking to former prime minister Julia Gillard on A Podcast of One's Own, the Canberra senator revealed that she struggled to cope with "the unfairness of it all" as a grieving mother-to-be.

"We were actually on holiday at the time, we were just having a weekend off down the coast," she said.

Mr Seaman had gone out on a bike ride and was hit by an unlicensed driver aged in her late 80s - who "shouldn't have been behind [the wheel] of a car" - at more than 100km an hour.

Senator Gallagher said her then fiancé had been "a very good cyclist" and an experienced road cyclist, but had stood "no chance".

"I was out looking for him. It was back in the days before mobile phones. He just never came back, and I actually heard about it whilst driving around looking for him on the radio," she told Ms Gillard.

"I'd tuned to the local radio station and they'd put a call out to say police were looking for somebody who knew this cyclist who'd been killed, and so that's how I actually heard about it."

Senator Gallagher, who holds three portfolios in the Albanese government as Minister for Finance, Women and the Public Service, described the loss as a moment that changed her forever and taught her "empathy and understanding".

She recounted her mental health struggles as she adjusted to the change from her "happy and privileged life" to that of "a single parent on a supporting mothers payment, finding it difficult to work, having to find a new place to live".

"I was very difficult to be around for my family and friends who wanted to care for me," the senator said.

"I just, I couldn't allow that, and so I spent, for the rest of the pregnancy in a pretty bad shape and had ... some pretty serious psychiatric interventions. I really did go into the bottom of the bottom."

Senator Gallagher said it was only when she was towards the end of her pregnancy that a doctor's stern advice brought a moment of clarity that enabled her to "get myself back together".

"He said 'You're going to have a baby in six weeks and you're not going to be able to care for it' ... I thought, 'I can't now lose the baby' ... I thought 'Right, this is, this is for real now'."

Starting antidepressants "pulled me out of [the] hole I was in, and by the time Abby was born, there was no one who was going to take that baby from me," she said.

Mr Seaman was a well-known member of the ACT branch of the Australian Labor Party, and the local trade union movement supported Senator Gallagher after his death.

"He'd sort of got me more active than I had been before," she said.

She went on to enter politics, first in the ACT government where she rose to chief minister in 2011, then as a federal senator for Canberra from 2014 until today.