Simplicity's claim was based off the present value of 45 years worth of paying its KiwiSaver fees relative to a higher average KiwiSaver fee.
But the FMA said the assumptions Simplicity relied on were not reasonable, particularly that there would be no changes to fees over a 45-year period, given there had been significant recent changes in fees charged by KiwiSaver providers.
The assumptions underpinning the claim were also not prominently disclosed in advertising material so the public was unable to query the assumptions.
The advertisements appeared on TV channels, Facebook, YouTube, billboards, internet display banners and Simplicity's website between August to October last year.
Paul Gregory, FMA director of investment management, said advertising could strongly influence investors' decision-making.
"It is vital that providers ensure their marketing materials are factually accurate and don't mislead.
"The direction holds Simplicity accountable to investors and means we have additional responses available if Simplicity does not make the necessary improvements or fails to comply with the direction order."
He acknowledged Simplicity had withdrawn the campaign promptly, accepted responsibly and engaged with the FMA constructively, without defensiveness.
"This case sends a signal to the financial services sector that we will continue to use our powers to sanction providers who make misleading claims in their advertising, as set out in our guidance."
A Simplicity spokeswoman said it was not making any further comments.