A Sydney burger chain has been accused of trading off the "legendary reputation" of the American fast food outlet In-N-Out Burger, in a Federal Court food fight years in the making.

The California-based In-N-Out Burgers Inc launched Federal Court proceedings in October 2017 against Hashtag Burgers, which is headquartered in Coogee and operates Down N' Out restaurants in the Sydney CBD and Ryde in the city's north-west. The three-day trial started on Monday.

In-N-Out Burger claims the owners of Sydney's Down N' Out restaurants are infringing their trademarks.

In his opening address to the court, In-N-Out's barrister, Chris Burgess, said Down N' Out was "riding on the impressive coat-tail" of the US burger chain's trademarks, branding and reputation.

"They are still clinging to In-N-Out's coat-tails," he said, adding that it was clear "that they won't let go".