The Albanese government plans to lift the tax rate on earnings for superannuation balances over $3 million from 15 per cent to 30 per cent from mid-2025.
It will impact about 80,000 Australians and is expected to raise $2 billion in the first full year and $3.2 billion over five years, which will go towards balancing the federal budget rather than being spent.
NAB chief Ross McEwan said $3 million was "a lot of money to have in a super fund".
"I'm sure I'll put myself out there and people will say, 'He should never have said that'," Mr McEwan told ABC radio on Friday.
"But I think $3 million is a lot of money."
He said the decision would not impact 99.5 per cent of Australians.
"It will affect a small group of people who had a huge amount of money in those funds - I'm sure they'll find other things to do with it," he said.
"So that's just a reality of where we are. We're all going to have to play our part to get this economy back into shape, and get the debt down."
The Financial Services Council found in an analysis of tax office data that over 500,000 taxpayers would be impacted if the change is not indexed, including 204,000 people under the age of 30.
"Leaving the cap stuck at $3 million will mean that in today's dollars a 30-year-old will have a real cap of around $1 million, calling into question the intergenerational fairness of an unindexed cap," council CEO Blake Briggs said.
"Caps in the superannuation system are indexed to ensure generational fairness."
Opposition Leader Peter Dutton, whose coalition would repeal the changes in government, said the government had come up with a "crazy plan".
"It's going to cascade onto many other superannuants - there's no question about that," he told Nine's Today show.
Deputy Prime Minister Richard Marles said it was an important change to make the system sustainable.
"We inherited a budget from Peter and his crew, which was a trillion dollars in debt and there was nothing to show for it and we need to be responsible and that's what we're trying to do," Mr Marles said.
Meanwhile, there are warnings owners of small to medium sized businesses could be adversely impacted by the change.
RSM Australia's director of financial services, Grace Bacon, said many people managed their superannuation within their business structure.
"ATO data shows that more than one-fifth of Australia's self-managed super funds (as of 2020/21) sit in the $1-2 million asset range and it's not hard to see growth in the value of their property or assets flipping many of these above the $3 million cap," she said.
"This includes farmers, and suburban and regional SMEs – asset-rich, cash-poor in many cases - who have commercial property in their self-managed super funds which they rent for their businesses."
But she said it was important people did not rush to make changes to their super until the government completed its consultation period and released the final legislation.
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