"When you see the HYEFU numbers, what you'll see is the budget surpluses start hockey-sticking up and they start getting quite big," he said.
Pressed again on whether the Government could 'do it all', he said: "I think when you see the numbers next week, that's what you'll think as well."
He didn't quite use the word "bigly", but there were a few Trump-like assurances along the lines of 'believe me, when you see the size of my surpluses you won't be asking these questions'.
Referring to Treasury's forecasts and its 26 years of adherence to the Public Finance Act to give an honest account of the fiscal outlook is a perfectly rational thing for a prime minister to do.
Treasury's record as a forecaster is among the best in the world and the act was a pioneer in being designed to stop politicians cooking or hiding the true state of the Government's books before elections - as happened several times in the 1980s.
But Key's reliance on those forecasts for some big talking certainly jarred just two days after he rubbished the Treasury's forecasts in its Long Term Fiscal Outlook delivered last week. Under the same Public Finance Act, the Treasury has to forecast at least once every four years what the Government's books will look like by 2060 if it continues on with its current settings.
Treasury pointed out for the third time in the current National Government that an ageing population allied to the current settings for New Zealand Superannuation would blow out the Crown's net debt to over 200 per cent of GDP by 2060 from 25 per cent now.
Treasury again suggested the Government extend the age of eligibility for New Zealand Superannuation to 67 and move to a less generous form of indexation than the current one linked to the average wage. The trouble is Mr Key has promised to resign if either one of those two changes are made.
So his response this time was to say those particular long term Treasury forecasts were not reliable, and his proof was that the short term ones were not reliable either.
"The cool thing is Treasury can't get their predictions right in 44 days, let alone 44 years. They constantly get it wrong," Key said.
He was referring to Treasury's forecasts before the Budgets in May 2015 and May 2016, which were for small deficits when the Government had targeted surpluses. Eventually, the final numbers registered small surpluses, but not before causing some embarrassment for the Government.
Key had not forgotten when he was asked if he was betting his legacy on Treasury getting its long term forecasts wrong.
"I'm telling you it's a load of nonsense, because they can't get predictions in 44 days right, let alone in 44 years."
It is extraordinary for the Prime Minister to call the Treasury's short and long term forecasts a "load of nonsense" and yet rely on the medium term ones to promise all manner of riches to voters on the eve of an election.
Treasury's forecasts are worthy of respect. Key may well be right that the Government can afford it all for the next few years, but if that was the case it should also address those longer term challenges evident in the Treasury's long term forecasts.
One way to credibly address the issue is to use the good times now to prepare for the longer term challenges. As Labour's finance minister Michael Cullen resisted Key's calls from 2005 to 2008 for big tax cuts and instead set up the New Zealand Superannuation Fund and KiwiSaver.
That was a sensible way to address those long term challenges, although Cullen also fell short by not dealing with the elephant in the room of those unsustainable New Zealand Superannuation settings.
If there is enough in the surpluses to 'have it all', that 'all' should include resuming contributions to the New Zealand Superannuation Fund, which Key said in 2009 the Government could not afford. He should also allow a proper bi-partisan debate about those pension settings around the age of eligibility and indexation.
That would be the exact opposite of a "load of nonsense".
