National MP for Wairarapa Alastair Scott joined the chorus describing Key's announcement as a "big surprise".
"I thought he would go in and want to win a fourth term but I take his point if he was planning to leave anyway next term, he's doing the right thing leaving now.
"He will leave a huge gap and some big shoes to fill."
In terms of timing, he said it would give the new leader time to get their feet under the table and give the public some confidence.
As to who would step in, Scott said it was not just a case of a new leader, but the important role of deputy prime minister to be decided as well.