A Sydney burger chain passed itself off as the "legendary" US fast food outlet In-N-Out Burger when it operated as "Down N' Out", the Federal Court has found.

Hashtag Burgers, which runs the Sydney chain, was sued in 2017 after the California-based restaurant took exception to its name, similar yellow arrow logo, and use of In-N-Out's registered trademarks "animal style" and "protein style".

Food fight: In-N-Out sued Down N' Out in the Federal Court.

The trademarks, which form part of In-N-Out's "secret menu", are descriptions for burgers or fries with added fillings and a burger wrapped in lettuce instead of bread.

Down N' Out opened as a pop-up in Sydney in 2016, originally styling its name as "Down-N-Out", and has since had a presence in the CBD, Penrith, Wollongong, Crows Nest, Ryde and the Gold Coast.