A FAULTY switch and instruction manuals written entirely in Japanese have been blamed in court for why a ship owned by conservation group Sea Shepherd dropped up to 500 litres of diesel into the Trinity Inlet.

The environmental organisation, whose Australian arm is chaired by former politician Bob Brown, yesterday pleaded guilty to the marine pollution offence in the Cairns Magistrates Court.

Defence barrister Tracy Fantin admitted the irony of the charge given Sea Shepherd's charter to protect and preserve marine life around the world.

She said the spill, which took place while the ship was moored alongside a wharf at the Cairns Port on October 13, 2012, happened after a crew member named Gabor Nosty failed to manually flick the "low level" switch during a fuel transfer, despite being aware the switch was faulty.

The court heard Sea Shepherd Australia had only bought the boat, named New Atlantis, from Japan a week earlier and had yet to translate signage and manuals or repair the switch.

According to court documents, crew members had been given basic handover information but the chief engineer had to work out the ship's systems "by his own devices" due to instruction manuals and other materials all being in Japanese.

All crew members were volunteers and were either German, Dutch or American.

The court was told a passerby noticed diesel flowing into the sea about 6.30pm and tried to alert crew members before notifying the master of a ship moored alongside who boarded the New Atlantis and told them.

"She noticed a strong smell of diesel fuel and saw liquid running from the New Atlantis into the water," the court document read.

"The smell was so strong the passerby had to put a jumper over her nose ... "

It was estimated between 100 and 500 litres spilled out.

Magistrate Kevin Priestly called the amount "not insignificant" and questioned why a crewman was performing the fuel transfer and not the chief engineer.

Department of Transport and Main Roads prosecutor Anne Roseler called for a penalty of between $15,000 and $30,000. Mr Priestly adjourned the decision to a date to be fixed. Charges against Mr Nosty were dropped.

