Watch it live here.
He was grilled about his former ministers on AM.
"One of the hardest things I had to do was to tell him that I was looking to move on," Robertson said.
Robertson will now take up the role of vice-chancellor at Otago University.
Here are some facts about his replacement.
He's got a new job.
Despite being deputy leader, he was never Deputy Prime Minister.
"I thought about it and decided that was the right thing to do."
"There's an expectation that the Prime Minister would set a high standard."
