The biggest spying threat facing Australia is the United States, one of the world's top cyber security experts says.

French-born, Sydney-based Jonathan Brossard, whose stable of clients includes the Commonwealth Bank and European car manufacturers, says the US may be a close ally but it would wrong to think the US was not spying on Australia.

Cyber surveillance: It would wrong to think the US was not spying on Australia, says a leading security expert. Credit:AP

"They are the biggest threat," the chief executive of Toucan System said.

"Australia is part of the Five Eyes and all that and there is a strong belief that allies are not spying on us.