But Mr Banks said Mr Dunne's claims that the policy would let Labour "off the hook" from its policy of raising the age of eligibility, "shows a lack of understanding about the effects of this policy".
"The reason Labour pledged to raise the age is to ensure the cost of New Zealand super remains affordable. Mr Dunne's policy fails to do that."
"Rather than just debate the issue of Flexi-Super, ACT would like to see all parties engage in meaningful discussion about the long-term issue of NZ Super affordability.
Raising the age of eligibility would be a good first step," Mr Banks said.
In its long term financial forecast released last month, Treasury said raising the age of eligibility for NZ Super was one of a range of policies the current and future Government's should consider to avoid a long term debt blowout.
But Mr Key this morning said Treasury would always make those recommendations, "because they would always recommend anything that cuts costs".
"But you go and work the numbers of raising the age from 65 to 67, in fact as the long term fiscal forecast proves, it is far less effective than changing the link to average wages and all the other things.
"Everybody argues about the age of eligibility of NZ Super being raised who has a serious debate about, actually in the next breath says you've got to make it less generous and you've got to means-test it, and once you get into that you're into a very different space."
"If we want to have an open honest discussion about it I'm more than happy to do that but anyone who thinks that raising the age of super will fix the economic issues they are dreaming, they will not.
Mr Key said the cost of NZ Super was currently 4.6 per cent of GDP compared to 7.5 per cent back in the 1970s.
"Is it really that big a deal? Well that's not what the 40 year forecast says. We've built it into our 20 year forecasts at its current funding.
"If you really want to say the magical rate is to keep NZ Super at the what it currently costs today then you have to means test, you have to de-link from average wages and you have to raise the age.
"If any political party wants to have that political debate with me on the campaign trail, bring it on."