Re-referencing the quarterly Consumer Price Index

As of the December 2025 release of the Consumer Price Index (CPI), the previously published quarterly CPI index number series have been re-referenced. The updated reference period for the quarterly series aligns with the reference period of the new monthly CPI series. The quarterly index numbers are now presented on an index reference period of September month 2025 = 100.00 and published to two decimal places. Previously, the index reference period was 2011-12 = 100.0 and index numbers were published to one decimal place. 

While re-referencing has changed the levels of the quarterly CPI index numbers, the percentage movements of the indexes have not been revised, aside from minor impacts of rounding. Changes to percentage movements which arise from the effects of rounding do not constitute a revision. These rounding effects are larger in earlier periods of the time series, due to the lower values of the index numbers.

Conversion factors are published in the December 2025 release of the CPI as ‘Quarterly CPI - September 2025 conversion factors’ in the ‘Data downloads’ section of this product. These factors enable users to convert the previously published quarterly time series to the new September month 2025 index reference period, or to convert index numbers on the new index reference period to the old 2011-12 reference period.

Each index series and capital city has its own conversion factor. For example, the conversion factor for the Clothing and footwear group for Perth will differ from the factor for the Health group for Perth, and from the Clothing and footwear group for Sydney.

Updating the index reference period results in the level of the index changing for the entire length of the time series. Chart 1 shows how the level of the quarterly index for the CPI has shifted down with the new reference period of September month 2025 = 100.00. While the level of the index has changed, the slope of the index is unchanged. This reflects the fact that the quarterly movements are preserved and do not change as a result of updating the index reference period.

Chart 2 compares the quarterly movements of the CPI with the old and new index reference periods. The quarterly movements are the same, except for some differences of 0.1 percentage points. These differences are due to rounding and are not considered a revision of the quarterly movement.

The following examples demonstrate the method used by the ABS to calculate quarterly conversion factors to convert from the old (2011-12 = 100.0) to the new (September 2025 = 100.00) index reference period and vice versa.

The conversion of index numbers from the old index reference period to the new index reference period involves a rescaling of the index numbers. The conversion factors that are applied are calculated by obtaining the ratio of the September quarter 2025 index numbers on the new index reference period (September month 2025 = 100.00) to the old index reference period (2011-12 = 100.0). 

To convert the index numbers to the new reference period (September month 2025), the conversion factor for the September quarter 2025 is calculated as follows:

Conversion factor = quarterly index on the new reference period / quarterly index on the old reference period = 99.73 / 143.6 = 0.6945. 

The factors are used to convert the index numbers from the old reference period (2011-12) to the new reference period (September month 2025) for all earlier periods. For example, the index number for the June quarter 2025 (index reference period September month 2025 = 100.0) = 141.7 x 0.6945 = 98.41

This conversion factor may be used to convert any historical All groups CPI, weighted average of eight capital cities index number to the new index reference period. Different conversion factors are required for each index series.

Note that the conversion factors used by the ABS are not rounded and are also calculated from non-rounded index values. This may contribute to slight differences between user calculations and ABS published numbers.

Similarly, converting index numbers on the new index reference period back to the old index reference period also requires rescaling of the index numbers. The conversion factors that should be applied are obtained by taking the inverse of the previously described conversion factor.

Using the example above, a conversion factor is calculated as follows:

Conversion factor = 1 / conversion factor to convert to the new index reference period = 1 / 0.6945 = 1.4399

Index number for the September quarter 2025 (index reference period 2011-12 = 100.0) = 99.73 x 1.4399 = 143.6

Index number for the June quarter 2025 (index reference period 2011-12 = 100.0) = 98.41 x 1.4399 = 141.7

Quarterly seasonally adjusted estimates, including the Weighted median and Trimmed mean, are compiled on the pre-October 2025 collection frequency. More information about the pre-October 2025 collection frequency indexes can be found in the information paper Quarterly Trimmed mean (pre-October 2025 compilation basis) explanatory note. These series have not been re-referenced and will remain on the 2011-12 index reference period.

Index numbers are published to two decimal places. Percentage changes are published to one decimal place and calculated from the rounded index numbers. Prior to the October 2025 release of the CPI, percentage changes were calculated from index numbers rounded to one decimal place.

These changes, along with the effect of rounding, may lead to quarterly percentage movements differing slightly to those previously published. These differences do not constitute a revision. 

There may also be slight differences between user calculations and published values due to the greater level of precision of the values used in calculations performed by the ABS. In particular, the conversion factors used by the ABS to calculate the re-referenced index values are based on unrounded index numbers.