Shell says it's business as usual at the Corio refinery, despite the decision to close its only other Australian refining operation.

The oil giant has announced the Clyde refinery in Sydney’s west will close in September, at a cost of 700 jobs.

Unions say the move is a mistake and will make Australia dependent on oil imported from other countries.

The Clyde operation currently produces around 40 per cent of NSW petrol supplies.

The move has sparked fears the company may be rethinking the future of its Australian refinery operations but Shell says there are no plans to close the Corio plant.

It says it’s still investing in the Geelong refinery and surrounding infrastructure.