"They basically run the company," said one source. "So that was a big deal."
Their position was made known to the MediaWorks board at its meeting on Monday, which also received a similar letter from Barry's 6pm co-host Mike McRoberts.
London-based Jonas Mitzschke, senior vice president of MediaWorks' owner, Oaktree Capital, was said to be particularly perturbed about the threats. He was here for a scheduled visit to coincide with the board meeting.
"The planets basically aligned," said one source. "Everyone knew it was now or never to make the move."
It is thought Matthews, a former Fairfax Australia boss and only appointed to the MediaWorks board last month, will be mining his contacts in the hunt for a replacement for Weldon.
And although the search is expected to go international, sources said a familiar hand on the tiller could be what the company needs for stability. Palmer would be the favoured internal candidate.
"She knows the business well and is a good people person. She would have the support of the key radio and TV executives," said a source.
One source speculated an internal candidate could fill the role for a year or so to settle things down.
"You know, much of the strategy was right, but Weldon was just not the right person to deliver it.
"It will be hard to find the right person to replace him, though.
"You can't make the same mistake again. And you can't bring in someone alien to media culture. At least Matthews has some idea of the challenges facing the industry."
In another development, the company's new head of news will now start the job earlier.
Hal Crawford, editor in chief at ninemsn in Australia, was due to start in July, but that has been brought forward a month to help rebuild the newsroom.
One staffer said he would have a hard job. "Everyone is happy Weldon has gone but there is still a bit of trepidation about how Hal will go."
Meanwhile, there are growing calls for Barry to get her own show at TVNZ and sources told the Herald on Sunday she has been talking to the network about revamping Breakfast. If that went ahead Barry would be up against her old workmate Paul Henry.
Barry celebrated Weldon's departure by carting a case of Moet into TV3. One source said: "They still had a bulletin to get out but there wasn't much work done that day."
McGeoch and Matthews could not be reached for comment.