ACC says it is plausible that interest rates will stay lower over the long term, eroding the state-owned accident insurer's theoretical returns relative to the cost of future claims.
The corporation's returns relative to future claims are benchmarked to government bonds, which are trading at near record low yields as the central banks keep their key interest rates near zero.
New Zealand 10-year government bonds are yielding 3.45 per cent, less than half the 8 per cent rate in 1992. Respondents in a Reserve Bank survey last month said they expect the yield to rise to 4.1 per cent by March next year.
"We certainly think real interest rates could be lower than has been usual in the last 20 years," said ACC general manager of actuarial and risk Herwig Raubal.
"Lower long-term interest rates are a plausible scenario."