Runaway population growth in Melbourne's north-west has put plans for a rail line to Melbourne Airport in jeopardy, just as calls by the airport for a rail connection grow more urgent.

The Andrews government has been warned it will need to invest billions on a new rail link between the airport and the city in future years, or hope the Tullamarine Freeway can continue to carry the load, despite warnings from the airport that it faces gridlock within a decade if a rail line is not built.

Credit:Chris Hopkins

Melbourne Airport believes a rail link is needed within 10 to 15 years.

The government's $10.9 billion Melbourne Metro tunnel was designed to create capacity for Melbourne's rail system to support a link to Tullamarine, with airport trains proposed to run via the planned Metro tunnel some time after it opens in 2026.