A Darwin man has been fined $1,500 for fishing at an Aboriginal sacred site.

The court heard 56-year-old Rudolf Brkic set his nets in the mouth of the Blyth River in Arnhem Land, north-east of Darwin.

Fisheries officers monitoring Brkic then found the nets that were marked with the professional fisherman's name and registration number.

Today his lawyer Vanessa Farmer said there were no markings, the beach was open and there was nothing to say it was a sacred site.

Magistrate Greg Cavanagh fined Brkic $1,500 and said business operators ought to keep themselves informed about where sacred sites were.

The offence carries a maximum penalty of $20,000.