Sydney should be 3 cities: Lucy Turnbull

Sydney should be "re-imagined" as three great cities, Greater Sydney Commissioner and former Lord Mayor Lucy Turnbull has argued.

The three cities would be Eastern Harbour City, the Central Parramatta River City and a Western City around the new Badgerys Creek airport under Ms Turnbull's proposal.

She argues the change is necessary to keep commute times within an hour while the country's biggest city expands westwards and with Sydney's population expected to almost double within 40 years.

"It's time for a change of perspective and a change in the way we all imagine greater Sydney. Greater Sydney needs a true city at its centre, close to its people," she said in a statement on Thursday.

She said the three cities approach "gives a clear picture of how people can realistically achieve the goal of being able to live within 30 minutes of where they work, study and play. This makes our city more liveable, productive and sustainable for everyone."