Advocates for elderly people say a proposal for flexible superannuation rates could lead some pensioners into a life of grinding poverty.
The Government is seeking feedback on a policy from United Future allowing people to claim a reduced pension from the age of 60 or a higher rate if they defer retiring until they are 70.
The payment at the current age of eligibility, 65, would remain the same.
Age Concern says the reduced rate would need to be set at an adequate level or people could have miserable lives in retirement.
Co-director of the Retirement Policy and Research Centre at Auckland University, Michael Littlewood, says the idea is also potentially expensive, lacks detail and would complicate a simple system.
"Those with shorter life expectancies will presumably choose to take it up earlier and therefore get more pension than they would otherwise have got.
"People who can afford to defer it to age 70 are likely to be those on higher incomes and typically those have better health and therefore last longer on average and will get a higher pension for longer."
Co-chair of the group Auckland Action Against Poverty, Sue Bradford, says she has little faith the state would pay extra benefits to those struggling on the reduced rate.
"If I haven't got a job and I'm at the mercy of work and income. Or if I'm struggling to hold down three part time, very poorly paid jobs while also trying to care for my family would I rather be on a low pension? Yes; but what that condemns them into is the rest of their life being on this poverty stricken level of income."
Ms Bradford says it would make the gap between the rich and poor in retirement even wider.
Auckland University retirement expert Susan St John says the proposal favours the most well off.
She says there are better ways to help those who are currently struggling to make it to 65 in the workforce, including changes to welfare.
Plan has merit - Key
Prime Minister John Key has staked his political career on maintaining both the age of eligibility and the current entitlements.
He says introducing the more flexible policy would not break that promise as it would be entirely voluntary and people could continue to begin receiving payments at the age of 65.
Mr Key says the plan is worthy of discussion and he is interested in hearing feedback.
Labour Party finance spokesperson David Parker says the policy does nothing to address the looming financial cost of the current system.
He says Labour is commited to raising the age of eligibility to 67 over the next ten years.
Public submissions United Future's proposal close in October.