"The AAAf fund credit quality rating reflects Standard & Poor's opinion that the fund's portfolio holdings provide extremely strong protection against losses from credit default," S&P said.
The fund invests in high-quality short-term securities issued by Australian deposit-taking institutions covered by the Australian Government Guarantee Scheme.
It is managed by UBS Global Asset Management (Australia) and seeks to match the performance of the UBS Bank Bill Index - before management fees and a fee paid for the Australian government guarantee - when measured over rolling 12-month periods.
"We consider UBS' research capabilities to be very strong," S&P fund analyst Lisa Barrett said.