The Financial Markets Authority (FMA) is renewing its push to lower KiwiSaver fees amid concerns about excessive charges.
The regulator is concerned fees may have a negative effect on balances, as well as a disproportionate effect on KiwiSavers with low balances.
"The members' interests must be put first and they must be treated equitably," it said.
"A membership fee that erodes a member's low balance is likely inconsistent with these duties."
It was also looking at management costs and whether fees have kept pace with the shift from higher-cost active fund management to lower-cost passive funds.
"Independent research has shown that the global trend is a decrease in fees for passive funds, and suggests that fees charged by KiwiSaver providers are high compared to broadly similar funds in the UK.
"Benefits of scale - at least for the larger providers - do not appear to have been passed on to KiwiSaver members."
The FMA said it had a variety of regulatory tools available to monitor and enforce those relevant obligations.
Consultation on the issue closes in 14 December, with final guidelines on fees to be available early next year.