"It's concerning ordinary KiwiSavers might rely on these projections," he said.
Stubbs said providers should either use a standard set of assumptions set by the government or have an actuary sign off on the figures.
The Ministry of Business, Innovation and Employment is in the process of finalising how KiwiSaver retirement savings and income projections will be calculated for inclusion in annual member statements.
If approved by Cabinet these changes would become compulsory for KiwiSaver provider statements from 2020.
Stubbs said providers should have to use a standard set of assumptions in all marketing used to forecast a member's future wealth.
He also urged providers to use figures that were both before and after inflation to show people the future purchasing of their money and tax should be applied at the member's rate rather than using the top rate of 28 per cent.