"How many times do the National Party and its caucus need to be told to stop the leaking?
"Why are they so stupid that after a cataclysmic defeat in 2020, they haven't got that basic message?"
He said he had never seen a party self-destruct in such a way as National had and pointed the finger at bad candidate selection by the board, saying party president Peter Goodfellow should consider standing down.
"They are the ones who I think are responsible for many of the embarrassments that are haunting the National Party now. They've let the side down. They need to clean out the board and put some people in there who actually care for the National Party and love its traditions rather than being careerists."
He pointed to the cases of former Rangitata MP Andrew Falloon and Upper Harbour candidate Jake Bezzant, both of whom were accused of inappropriate use of social media or texting.
"How did they ever happen? You can't blame the caucus if people come in. But the President and the board, their most important function is to choose good people for the party.
"I think Peter Goodfellow should really consider his future. He's been there for 12 years, and every dog has his day and maybe it's time for him to move on."
A spokesman for the National Party said Goodfellow did not wish to comment.
Finlayson said he did not believe the damage to National was terminal and his own loyalty to the party he has long been a member of "is not in question."
"But it's because I care for the National Party that I've been so appalled at the candidate selections."
He said after a review in 2002, candidates' colleges were set up to allow the party to have a good look at candidates and for candidates to decide if they were a good fit.
"If the rules were followed, and if there was proper vetting the National Party wouldn't have these problems. If you're going to be given the opportunity to represent the National Party in a blue ribbon seat, then you're expected to be a person who will measure up."
Judith Collins declined to comment, but speaking on RNZ this morning she said Finlayson was clearly frustrated by cases such as Falloon and Bezzant. "But they have been dealt with."
She said part of the party's review and proposed rules changes included toughing up selections. "One of the things we need to improve is the way we select candidates so we are selecting on character and strength, not just glossy CVs."
She said Finlayson had left two leaders ago. "I know he was disappointed at leaving … he's found a new career and I think we wish him well."