Asked why it was not a decision for Muller to make, she said "obviously he's a member of caucus, so he'll be part of that decision-making" .
The comments appear to raise the prospect Collins could ask caucus to force Muller out earlier than 2023.
Muller is an electorate MP and while caucus can suspend or expel him, it cannot force him to leave Parliament unless Collins invokes the waka jumping legislation - a law National is strongly opposed to.
Muller is currently on five weeks' leave, caring for his wife after surgery.
Asked if she was becoming more ruthless as leader, Collins said "oh, I think that's a very harsh word".
She said she was "very confident" caucus was focused on what it should be. "Oppositions have to stay focused on the things that matter.
"I'm making that very clear. When it comes to talking about things like leaks from caucus, those are things for me to deal with."
Harete Hipango and Barbara Kuriger, the MPs at the centre of the saga, also fronted to media but both refused to comment on the issue. Both said they would not talk about caucus matters.
The NZ Herald understands Barbara Kuriger was the MP who told Collins about it – she was in a car with Muller at the time he spoke to Newsroom.
Hipango was also asked about claims of inappropriate spending in the last term of Government for items of furniture she was made to repay Parliamentary Service for.
Collins has put that down to a "coding error" by a staffer, but Hipango refused to explain that error, saying it was dealt with appropriately by Parliamentary Service and there was nothing improper in her spending.
New MP Christopher Luxon would not talk about the ructions, but said Collins was "a great leader and she's making us focus on the things that matter".
Asked if he would rule out running for leader, he said "Judith is doing an amazing job" and he had come in new and had a lot to learn.
Asked how to stop MPs from leaking, he said it was up to each individual to decide how to manage themselves "and how they play together as a team".
He said it was a decision for a leader to make on how to deal with leakers.
Former leader Simon Bridges, who was rolled by Muller in 2020, said Muller had "a bunch of admirable qualities".
"Politics is tough, it's a long time away from family, long hours and he's made his decision. I wish him and his family well."
One of the few MPs with kind words about Muller was party whip Matt Doocey, who came into Parliament in the same year as Muller. Doocey would not talk about the reasons for Muller's resignation, but said it had been a "tough day" having to farewell "one of my good mates".
"That hurts, but we respect Todd's decision.
"I was a classmate of Todd's, class of 2020, and classes are a bit like a family really. You don't always get on or have the same views but what you do is experience the same milestones through your career. And it's sad he's made that announcement but I respect his announcement, and I also respect his request for some privacy as he's looking after his wife Michelle after a very serious back operation. I wish them all the best."
He insisted there was a "high trust environment" in caucus.
