"The offer came pretty much on Christmas Eve," he said.
"It's a job that I've thought about doing for a long, long time now. It's a fantastic role. There aren't many vehicles in journalism where you can rip in to some really substantial issues every day."
He resigned from RadioLive and TV3's parent company MediaWorks the same day - stepping down from his role as Third Degree co-host and walking away from an opportunity to present The Nation.
"They were interested in me presenting The Nation and I thought about that, they were good opportunities, but I just felt that this was something new and really attractive and so I went for that.
"I've done 10 years on TV now...and done a lot of stuff that I'm proud of, but I really felt it was time for a new challenge.
"I didn't want to try and spread myself too thinly, so I'm going to be concentrating on trying to do Morning Report the best I can."
Espiner said he expected to bring a different style to the role. "I've got to get in there and prove myself, that's all part of the challenge."
His good mate and Third Degree co-host Duncan Garner supported the move, he said.
"He loves his radio job and so he's excited for me. He knows what a great medium radio can be - it's intimate, flexible, there's not the sort of baggage that comes with TV."
MediaWorks director of news and current affairs Mark Jennings said Espiner had been an integral part of the ongoing success of TV3 news and current affairs.
"It's been a pleasure to have him on board. Morning Report is a unique role and a great opportunity, we sincerely wish him all the very best."
Espiner will continue in his role on 3rd Degree until he begins at RNZ in April.
A MediaWorks spokeswoman said an announcement regarding his replacement would be made in due course.
Espiner has previously been political editor for TVNZ and earlier wrote for the Sunday Star-Times.
Radio New Zealand chief executive Paul Thompson said Espiner's appointment would mean New Zealanders were well served in what was going to be an interesting election year.
"The public expects the country's business and political leaders to be held to account."