The Green Party says competition from a public KiwiSaver provider would drive down fees and costs for people paying into the scheme.
The party is proposing a public KiwiSaver option which would be managed by the Guardians of the New Zealand Superannuation Fund.
Greens' co-leader Russel Norman says a significant proportion of savings are eroded by fees and costs but that could be significantly reduced with a public KiwiSaver option.
"We'd be kind of bolting it on to the existing six default providers, putting a seventh default provider there with a lower cost structure", he says.
Dr Norman says competition from another default provider would also drive down costs.
National Party leader John Key says the Greens have raised issues worth looking at.
He says he agrees fees should be kept low by KiwiSaver providers but he does not think fund management should be outsourced to the Guardians of the Superannuantion Fund.
Labour Party leader Phil Goff says his party would have to look at the costs of setting up a public provider to make sure it wasn't prohibitive.
Mr Goff says he'd prefer in the first instance to see the promotion of companies already offering KiwiSaver more cheaply and more efficiently. He says some are already offering KiwiSaver with very low fees.
National says people would incorrectly assume a government-run KiwiSaver provider was guaranteeing their savings.