While Peters had "no regrets" about his decision to join with Labour, he said "what you see now of course is not my responsibility".
The Labour/New Zealand First coalition government was "dramatic ... in turning many of our failed systems around", including transport, and infrastructure in the provinces, Peters said.
Mackay asked if he regretted choosing Ardern over Bill English, when at least 60 per cent of New Zealand First voters favoured the National Party in 2017.
"The people who say we should have done this and we should have done that weren't in the negotiations," Peters said.
"You had a very, very arrogant National Party who just thought they were born to govern.
"They'd been in government for nine years, and you can't tell me one thing they did."
Another point in Mackay's open letter was that the current Government should look to history to see how to deal with the farming industry. He used Rogernomics as an example.
Mackay wrote that while Roger Douglas' radical economic restructuring in the 1980s "gutted provincial New Zealand" at the time, history proved he was "a visionary and the man most responsible for where New Zealand agriculture finds itself now - as the most sustainable farming nation on Earth".
Peters disagreed.
"I can remember the farmers being wall-to-wall facing a guy called Trevor de Cleene about what Labour had done to the farming community because Douglas [didn't] turn up."
Mackay insisted Douglas was eventually right, he just went "too hard and too early".
Again Peters disagreed, saying Douglas' "economic anarchy" was in stark contrast to the Australian government, where Bob Hawke and Paul Keating took an "incremental pathway to improvement".
"We were taking an absolute anarchic rebellion and throwing the baby out with the bathwater.
"What works with people is GDP and personal income improvement and that's where we've failed massively."
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Overall, Peters judged Mackay's open letter to be off the mark.
"Good try, but the wrong economic analysis."
Meanwhile, Peters said New Zealand's rural community taking to the streets for the Howl of a Protest was "inevitable" and would not have happened on his watch.
The Groundswell-organised protest was an example of how "things had become chaotic" for the Government, without Peters providing a "handbrake".
The ute tax would not have gone ahead if he had anything to do with it, Peter said.
"We're the ones that slammed their ute tax last year and stopped it. You all know that don't you?"
The Green Party pushed the tax once New Zealand First had left Parliament, Peters said.
He called the scheme a "farce".
"The very day the second hand ute subsidies…were notified by the Government - the price went up by that same amount."
Peters claimed purchasing a new ute would be far too expensive under the scheme.
"These things are so far out of the pocket of the ordinary person, it's not going to happen. It is a policy that seems to have been drafted by someone who couldn't organise a party in a brewery.
"That's the sad thing about it – and farmers know that."
Some saw expensive farm equipment as a luxury rather than a necessity, which showed a lack of understanding with the rural sector, Peters said.
"You see what happens when people get so out of contact with the driving economic force of this country – most compellingly proven during Covid-19 in 2020."
