"High single-digit returns from fixed interest in the future are unlikely, given that interest rates are so low," he said.
"For those who have a long time to retirement [at least 10 years or more], a more balanced approach with a tilt towards growth assets is likely to be the more appropriate option."
Douglas also warned investors off picking individual funds based on past performance.
"Choosing a KiwiSaver provider on the back of its five-year performance number alone is therefore likely to be a poor decision.
"We know that fund manager results swing around, and while a lot has happened over the past five years, this is still a comparatively short timeframe."
Douglas said KiwiSaver assets had grown at a phenomenal rate.
At the end of the first year of KiwiSaver just $954 million was invested. That has since grown to $12.1 billion, with more than two million members.
Best Performer over five years:
Milford Active Growth Fund 12 per cent per annum
Worst performer over five years:
Smartshares Growth Fund -5 per cent per annum