The Official Cash Rate (OCR) compound index simplifies the calculation of compound interest rates providing a standardised basis that is published by a recognised benchmark administrator. The OCR compound index is equivalent to a series of daily data representing the returns from a rolling unit of investment earning compound interest each day at the OCR. The change in the OCR Compound Index between any two dates can be used to calculate the interest rate payable over that period.
The interest rate payable over that period would be realised New Zealand Overnight Index Average (NZONIA) which is a term risk free rate. Realised NZONIA is a backward-looking rate based on the OCR, compounded daily in arrears over the relevant period.
More information and methodology can be found here.