For the Australian dollar and front-end yields, the reaction is likely to hinge on the quarterly trimmed mean, not the headline monthly print. A reading at or above the expected 1.0% in the quarter would run ahead of the pace implied by the RBA’s August forecasts and strengthen the case for a hike at the November 2 and 3 meeting. A softer core reading would give the RBA room to wait, even if fuel-driven headline inflation runs hot. A gap between the monthly and quarterly headline figures can trigger an early reaction that later reverses, as traders look past the headline to the core numbers. Oil remains the main driver of headline inflation, so energy prices will shape how hot the monthly figure runs in the months ahead.
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Australia’s monthly CPI now sets the headline, but the quarterly trimmed mean is still the number the RBA trusts most, and that makes it today’s main event.
Summary:
- Both the September monthly CPI and the September quarter CPI are due at 00:30 GMT (20:30 US Eastern on Tuesday, 11:30 Sydney).
- Since November 2025, the ABS has published a complete monthly CPI covering most of the basket every month, replacing the partial Monthly CPI Indicator.
- The monthly CPI is now the headline inflation measure and the benchmark for the RBA’s 2% to 3% target. The quarterly CPI is now the average of its three months.
- The RBA still uses the quarterly trimmed mean as its main underlying inflation gauge, while watching the newer monthly measures as their seasonal patterns settle.
- Major banks expect quarterly trimmed mean of around 1.0% q/q and 3.6% y/y, above the roughly 0.8% pace implied by the RBA’s August forecasts. Headline is seen at around 1.3% q/q and 3.9% y/y.
- The September monthly headline is expected to rise to around 4.2% y/y. It can run above the quarterly rate when inflation is building through the quarter.
Australia releases two inflation readings today, the September monthly CPI and the September quarter CPI, and understanding the difference between them is key to reading the market reaction. Both are due from the Australian Bureau of Statistics at 00:30 GMT, which is 20:30 US Eastern on Tuesday and 11:30 in Sydney.
Until late 2025, the quarterly CPI was Australia’s official inflation measure, and the monthly figure was only an indicator. That old Monthly CPI Indicator updated prices for only about half of the basket each month, with many services priced just once a quarter, so it was patchy and prone to swings. Markets watched it, but the Reserve Bank of Australia treated it as a rough guide.
That changed in November 2025, when the ABS began publishing a complete monthly CPI, starting with October 2025 data. It now collects prices for most goods and services every month, using sources such as web-scraped online prices, transaction records and government data, and it is published with seasonal adjustment. The monthly CPI is now the main measure of headline inflation, and its annual rate is the benchmark against the RBA’s 2% to 3% target.
The quarterly CPI has not disappeared. It is now built from the monthly data, with each quarter’s index the average of its three months, and published alongside the monthly figures in the March, June, September and December releases. So the quarterly headline has largely been replaced in importance, but it is now a by-product of the monthly series, not a separate survey.
The bigger reason the quarterly release still matters is underlying inflation. The RBA’s preferred gauge has long been the quarterly trimmed mean, which strips out the largest price rises and falls each quarter to show the core trend. The central bank still uses that quarterly measure as its main underlying gauge, and its forecasts are framed in quarterly trimmed mean terms, while it watches the newer monthly measures alongside it. The monthly trimmed mean has only around 18 months of history, and its seasonal patterns are still being established, so it can be noisier. The RBA plans to shift toward monthly measures as they bed in, with the ABS committed to producing the quarterly series until at least 2027.
That makes today’s quarterly trimmed mean the number most likely to move markets and shape the RBA’s November 2 and 3 meeting. Australia’s major banks broadly expect it to rise around 1.0% in the quarter and around 3.6% year on year, above the roughly 0.8% quarterly pace implied by the RBA’s August forecasts. On the headline side, quarterly CPI is seen up around 1.3% in the quarter and 3.9% year on year, while the September monthly figure is expected to rise to around 4.2% from 4.0%.
Those two headline numbers are not contradictory. The monthly figure compares September with September last year, while the quarterly figure compares the average of July, August and September with the same three months a year earlier. If inflation is rising through the quarter, the monthly rate will run above the quarterly one, which is what forecasters expect today.
For traders, the practical rule is that the monthly headline sets the tone, but the quarterly trimmed mean carries the most weight for the RBA, at least until the monthly measures earn the same trust.
This article was written by Eamonn Sheridan at investinglive.com.