"I encouraged Katy absolutely," he said from the Blue Mountains on Friday. "I told her to take the leap - take a deep breath and take the leap.

"I have no doubt my former colleagues in the Assembly would have felt a little wobble in the knee when Katy informed them she was going to nominate and will be leaving the assembly," he said.

He told her no one was indispensable and not to worry about letting down the ACT Labor team, but to have faith in her deputy Andrew Barr to take the reins.

"It's a hard decision and I sympathise with Katy. When I decided to leave … I was quite troubled about whether or not I was doing the right thing by the party," he said.

"I know Katy took all of this into account. I have absolutely no doubt that the largest issue in her mind when she took the decision … about whether to pursue a career in federal Parliament was loyalty to her Assembly colleagues and whether or not she was letting them down."

He had advised her to put faith in Mr Barr and the rest of the team.

"There will be an enormous gap. I'm not underplaying it, they're going to have to work hard. The acid's on Andrew and Simon [Corbell] as the two senior member of the cabinet. They're going to have to lift, and that's the challenge, but they have the capacity."

Mr Stanhope said there was no one else he could have fully supported as a Senate candidate, but Ms Gallagher had his "unhesitating, total and unequivocal support".

It was vital that ACT Labor put forward candidates capable of walking straight onto the federal frontbench, he said.

"They have to have obvious credentials for a ministry. They have to be a person you can look at and say, 'That person can take a frontbench, a cabinet position, in a future Labor government."

The only person with those credentials among possible candidates in the ACT was Ms Gallagher, who could match almost anyone on Labor's frontbench.

It would be a "travesty" if she didn't go straight into a frontbench job, he said.