A test of online KiwiSaver calculators has found there is nearly a million dollars difference between what they estimate a person would have at retirement.

A 20 year old who started out with a zero balance and saved 3 per cent of their $40k salary with an employer contribution of 3 per cent investing in a growth fund could have somewhere between $459,000 and $1.36 million at retirement depending on which calculator they used.

Sam Stubbs, managing director of low-cost KiwiSaver provider Simplicity which carried out the research, said he believed there was a risk that some investors would be lulled into a false sense of security when they saw the numbers.

"The numbers some of these 'calculators' produce would be good entries in a fantasy fiction competition, but bear no reality to what people will receive when they retire.

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"At worst some of these are dangerous mis-representations, luring KiwiSaver members into a false sense of security about their financial wellbeing."

One provider's calculation made no adjustments for tax despite all investors having to pay tax.

Others used wage inflation estimates above the current 10 year average of 1.7 per cent.

Stubbs said he had written to the Financial Markets Authority to draw its attention to the issue.

"We think all calculator assumptions should either be either verified by an actuary, or mandated by the FMA.

"Either way, KiwiSaver members need to be told what they could reasonably expect to have when they retire, not what a fund manager dreams up to make themselves look good."

How providers calculate what a person might have at retirement is under the spotlight at the moment as the Government is looking at including the information on annual statements as well as a figure for how much income that lump sum could give a person in retirement.

Chris Douglas, director of manager research ratings at Morningstar in New Zealand, said it worried him how the figures would be worked out.

"It is heavily reliant on assumptions. That type of thing really does worry me. I would hate for people to be complacent because they think the returns will be higher that they will be or the reverse: worried they won't have enough which causes stress and anxiety."

"It is very complicated."

Ayesha Scott, a senior finance lecturer at Auckland University of Technology, said forecasting needed to be done very carefully as there were no guarantees a person would end up with that money.

She said it would be better to give people a range rather than a set figure on annual statements.

"Then they can appreciate it could be higher or lower. If you give people a number they are going to target that number."