Read also:
• Understand your KiwiSaver options after 65
• Mary Holm: The low-down on high-risk KiwiSaver funds
•KiwiSaver should spread net privately
While Treasury was not concerned about concentration itself it warned that the growing significance of large banks could result in consumers getting a raw deal when it comes to fees.
"Concentration per se is not concerning as economies of scale exist in funds management which should, in theory, lead to cost reductions and efficiency gains.
"Financial capability of KiwiSaver members will be critical to ensuring the benefits of such economies of scale are captured by consumers.
"Certain trends, such as a growing significance of large banks, could detract from this and should be monitored to ensure that contestability in the market exists."
ANZ and ASB bank are the two largest KiwiSaver players in the market and between them manage around 44 per cent of the total asset pool.
Treasury found current fee levels were in the upper third for KiwiSaver compared to other similar countries and were "well above" the extremely low fees available in some markets.
Certain trends, such as a growing significance of large banks, could detract from this and should be monitored to ensure that contestability in the market exists.
It was also critical of how KiwiSaver funds have performed.
"In aggregate, the returns to members have not outperformed benchmarks chosen by us and are mixed compared to the investment performance of the Crown financial institutions."
It warned that KiwiSaver's weighting towards income assets rather than growth assets could lead to "less than optimal future retirement incomes."
Read the full report here: