New Zealand should increase KiwiSaver contributions and expand its coverage to improve the country's retirement income system, according to a global study.

The Melbourne Mercer Global Pension Index, which benchmarks retirement income systems around the world, included New Zealand for the first time this year giving it a "B" rating.

Although that might seem like an average result, no country received an A rating overall for its pension system.

The top-ranked country was Denmark, followed by the Netherlands and Australia which all received a B+.

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New Zealand ranked 9th behind Norway, Finland and Sweden, Singapore and Switzerland.

New Zealand performed well when it came to the integrity and adequacy of the system but was let down by its sustainability assessment getting an overall rating of 67.4.

The report said New Zealand's overall index value could be boosted by increasing the level of KiwiSaver contributions, raising the level of household savings, increasing the focus on getting an income from retirement savings rather than a lump sum and continuing to expand the coverage of KiwiSaver.

KiwiSaver has a minimum contribution rate of 3 per cent for employees and a further 3 per cent from their employers.

About 2.79 million people were signed up to the scheme as of the end of September and more than $40 billion is invested in it.

But about 500,000 working-age people are outside KiwiSaver and many who are in the scheme do not contribute anything to it.

In the run-up to the election, Labour said it wanted to make KiwiSaver universal and lift the contribution rate gradually to 4.5 per cent.

Labour finance spokesman Grant Robertson told the Herald last month that it those who were not in the scheme were largely on low incomes.

If elected, he said, the party would develop a scheme in conjunction with business and unions to bring low-income workers into the scheme in a sustainable way as wages rose.

Robertson said it would still exclude students, beneficiaries and self-employed people from having to join KiwiSaver.

The Labour-led government has committed to keeping the age of entitlement to New Zealand Superannuation at 65 and this is also part of its agreement with New Zealand First.