A second airport does not simply divide a market. It changes how growth finds a home.
Western Sydney International has a planning case that puts the airport at 19.3 million passengers by 2045. Where those passengers come from is a different question, and even the forecasts that agree on the total do not agree on its shape.
The familiar question is how much of WSI's traffic will come from Sydney Airport.
It assumes a fixed market waiting to be divided.
Australia's aviation map is already changing before WSI carries a passenger. Sydney Airport continued to grow. Other airports grew faster. Growth spread more widely across the national airport system.
WSI enters a system already in motion.
Volume tells us how large an airport becomes. Source tells us what its growth changes.
Australia's airport system began redistributing growth before WSI opens.
WSI's planning case gives that growth a new destination.It does not tell us where the passengers will come from.
The scale WSI's planning case assigns to the airport.
Traffic lost by Sydney Airport, a capture rate, or a measured division of demand between the two airports.
A passenger transferred from Sydney Airport, one recaptured from another gateway, one accommodated as Western Sydney grows, and one whose journey WSI enables all appear identically in the headline total. Their consequences are not.
Sydney Airport's national share declined in both domestic, including regional, and international traffic. The shift remains visible against both pre-COVID benchmarks, the year ending March 2019 and the 2017 to 2019 average. It holds when the comparison excludes Brisbane and Perth.
The shift extends beyond the rise of those two. Melbourne, Brisbane, Perth and the wider network have their own catchments, airline networks and reasons to grow. The data does not reduce their growth to traffic Sydney Airport could not accommodate.
Sydney Airport carried more domestic and regional passengers while its aircraft movements grew more slowly than the national market. It carried more passengers through each movement. Larger aircraft, fuller flights, or both could explain the change, and the signal survives the historical stress tests.
International traffic tells a different story. Sydney Airport's international share declined while growth spread across more Australian gateways, but the remaining difference falls within the known gap between the airline and airport datasets.
The same airport occupies different positions in different markets. In one the shift is clear and its mechanism is not. In the other the shift itself sits inside the margin between two data sources.
The published plans for Sydney Airport and WSI together reach 91.9 million passengers in 2045. A separate Sydney basin scenario extends pre-COVID national growth to approximately 89.0 million.
The totals sit 3.3% apart. From a distance they appear to describe the same future.
Look inside the total and the paths separate. The published plans carry 17% more domestic and regional passengers, and 7% fewer international passengers, than the independent scenario.
Four choices shape the analysis. Each marks a limit as much as a method.
The analysis measures airport shares from 2010 to 2026 before introducing WSI. A starting point cannot be created by an airport that has not yet opened.
The historical system is the baseline. WSI is the intervention.
Domestic and international traffic carry different signals. Separating them allows the analysis to identify those signals rather than average them into one airport-wide result.
The same airport can occupy different positions in different markets.
The published airport plans and independent Sydney basin scenario nearly reconcile on total passengers in 2045. Their domestic and international compositions do not. The mix reveals the divergence that the headline total conceals.
Agreement on scale is not agreement on shape.
The available historical airport data cannot divide WSI's future passengers between transfer from Sydney Airport, recapture from another gateway, accommodated growth, organic growth and induced demand. The analysis therefore does not manufacture a capture rate.
Knowing where the evidence stops is part of the result.
The analysis cannot tell us:
WSI's opening changes the evidence. Routes will begin, airlines will adjust, and Sydney Airport services may move, duplicate, grow or hold. Passenger-origin evidence will begin to separate transfer from recapture, accommodated growth and induced demand.
The analysis uses audited BITRE airport passenger and aircraft-movement data for rolling years ending March 2010 to 2026, domestic and international airline activity data, the Western Sydney International Master Plan 2025 to 2045, Sydney Airport's published planning endpoints, and the Sydney Airport Northern Winter 2025 slot report.
Each annual observation covers April of the preceding year through March of the stated year. National share equals an airport's passengers divided by all Australian airport passengers for the same market and period. An aircraft movement is one take-off or landing. Domestic covers Regular Public Transport including regional, counted at both airports, so figures represent airport activity rather than unique travellers.
Passenger change is separated into a contribution from aircraft movements and a contribution from passengers per movement using an exact two-factor Shapley decomposition. The two contributions sum to the observed change. Passengers per movement can rise through larger or fuller aircraft, which this data cannot separate, so no capacity conclusion is drawn.
Historical comparisons use the year ending March 2019 and the 2017 to 2019 average, with the 2010 to 2026 series as the primary view. The redistribution result was tested after excluding Brisbane and Perth. Passenger, movement and occupancy identities were reconciled across stages.
Concentration is measured as a Herfindahl index across the reported airports. Passenger activity became about 4% less concentrated at the national level, about 6% in domestic and regional, and about 3% in international, than in 2019.
The analysis uses published forecasts for arithmetic. It does not endorse them, and it creates no WSI capture rate, practical Sydney Airport capacity estimate, or independent allocation between the airports. It traces how growth shifted. It does not produce a new forecast.
A note on Sydney Airport's reported 95%. That figure tracks how often airlines operated the slots they held under the retention system, not how full the airport is. During Northern Winter 2025 Sydney Airport averaged about 50 gate movements per non-curfew hour, rising to 67 in the 9am hour, with airlines retaining 98.6% of domestic peak slot series. Pressure concentrates at particular times, not evenly across the day. This describes the operating setting and takes no position on the cap or curfew.
Figures current to the year ending March 2026 and updated at each BITRE release.
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