Bill English is expected to take over as Prime Minister and Steven Joyce is expected to take on the finance role. This will be confirmed next Monday.
Mr Reti said the decision did not affect his own political aspirations - he remained fully committed to aiming for a second term in the Whangarei electorate.
"We were on a caucus call 30 minutes before [the press conference] and that's when we found out as well," Mr Reti said. "I was surprised but I appreciated his honesty and I understand the reasons."
Mr Reti said there was "quietness on the call" and he could not gauge the mood within National until he met with his colleagues in Wellington tonight.
NZ First Leader and Northland MP Winston Peters claimed "hidden reasons" behind the resignation.
"The fact is that the economy is not in the healthy state that the Prime Minister has for so long claimed, and there are other issues which have caused this decision as well," he said.
Mr Peters did not elaborate on what these "other issues" might be in his initial statement.
Te Tai Tokerau Labour MP Kelvin Davis said Mr Key's resignation spelled opportunity for his own party.
"We were ready [for the election] anyway and this makes it a little more interesting," he said.
But it had little bearing on his own electorate, he said.
"I have to focus on what I've been doing and highlighting the fact that a lot has been achieved," he said. "We've heard the news, now we're getting on with making sure the people in our electorates are being served."
The Northern Advocate has been out and about In Whangarei today asking people for their thoughts on the PM's resignation. Reactions ranged from surprised joy to disappointment and those fearful about uncertainty.
People said they would remember him for steering the country through the GFC, pulling pony tails, the Trans-Pacific Partnership controversy and the flag referendum.
On Facebook, Advocate readers' reaction ranged from "good riddance" to "gutted, he's a good Kiwi man".