Gareth Morgan chose to announce his taxation policy outside John Key's "mansion" in Parnell, Auckland, on Wedneday

The nation's newest political party leader has a dim view of what the departing Prime Minister will be remembered for.

"Unfortunately John Key's legacy will be that he allowed the second round of expansion in inequality," economist, philanthropist and now aspiring-politician Gareth Morgan said.

Morgan launched the Opportunities Party (TOP) in November, with a vow to reduce the gap between New Zealand's haves and have nots.

He proposed a Robin Hood-style tax policy on Wednesday; a capital tax on home owners that will offset reductions in income tax across the board.

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​"I want to hold house prices - not collapse them - and give incomes a chance to catch up", he said.

This will boost the brass of 80 per cent of the population and leave the wealthier 20 per cent to "fund an increase in fairness in New Zealand", he estimated.

For the latter group, Morgan is the first to admit his policy proposal would be "like a cup of cold sick".

"This policy is one you do have to go away and think about - and then have a whisky and think about it again.... And then overcome your rage", he said.

"It's a very radical, fundamental reform for New Zealand."

TOP's next policy announcement would "probably be on immigration, probably next week", Morgan said.

As for a Universal Basic Income he hinted at earlier in the day, Morgan said it wouldn't be on the cards this time.

"[That would be in] phase two, if we're still about," he said.

He hoped to have all his current policies delivered by March next year, when he would glean the public's interest in TOP running in 2017's election.

"They might say 'thanks but no thanks Gareth, off to the beach you go'.... and I'd be happy with that too," he said.

He reckoned it would be worth joining the fray if 10,000 people had registered with TOP by March.

The idea for the Opportunities Party was seeded by a conversation Morgan had "a long time ago" with then opposition finance spokesman Michael Cullen.

"I went through this [tax proposal] with him and said 'so it makes sense, yeah?'" said Morgan.

"And [Cullen] said 'yeah'.

"And I said 'so you'll do it, right?'

"And he said 'no, no, no! Not in a thousand years ... because the public won't like it.'

"Then I said, 'but the public need to be taught as to why it's a good thing to do!'

"And he went, in that typical Cullen way, 'aha! That's your job.'"

Morgan said he sat on that revelation for a while, and this year decided it was the right time to start educating.

Under his taxation proposal, he would personally be whacked with an extra $350,000 per year if the tax rate was five per cent. He reeled off a slew of top-tier politicians who would face similar fates because "a tax loophole made it too easy to shore up capital in houses".

He said it was a sacrifice he and they should be willing to make: "This is about trying to improve the efficient use of our country's capital".

"The question I'm saying to New Zealand now is: Are you up for this? Or are you only paying lip service to your concern for inequality in New Zealand?"