Prime Minister John Key has said he is open to a discussion about United Future leader Peter Dunne's proposal for a 'flexible' New Zealand Superannuation scheme.

Finance Minister Bill English released a discussion document on the proposal on Monday. United Future and National agreed to issue the discussion document as part of their 2011 confidence and supply agreement.

The proposal is for New Zealand Superannuitants to choose whether to retire earlier on a slightly lower pension, or choose to retire later than the current regular retirement age of 65 on a slightly higher pension up until the age of 70.

Key told a post-cabinet news conference the proposal was fiscally neutral and did not conflict with his 2008 promise to resign if he ever changed the age of entitlement from 65 or changed the entitlement rate for couples of 66% of the average ordinary time wage.

He said the proposal was voluntary and therefore did not conflict with his promise and would not necessarily be something that would have to be put to the electorate in an election campaign before being adopted.

English said the discussion paper was designed to "test the public appetite" for providing more flexibility around when New Zealanders can opt to first receive New Zealand Superannuation.

“The discussion document is deliberately set at a high level and more detailed policy work would be required should the proposal be progressed," English said.

Public submissions on the proposal close on 11 October and English said he would report to Cabinet on the results of the consultation in November.

Dunne said a Fairfax Ipsos Mori poll earlier this year showed almost 50% supported the idea, which he said was almost more than twice the level of support for shifting the age of entitlement for superannuation to 67, or keeping it at 65.

“Flexi Super lets people choose for themselves when they want to take up superannuation – without being told by the government when they should or should not retire,” Dunne said.

Māori, Pasifika and other demographic groups with shorter life spans would benefit most from Flexi Super, he said.

(Updated with comments from English, Dunne and with link to full discussion document)