Tiwi Islander Dennis Tipakalippa is challenging the decision to allow Santos to drill eight wells in the Barossa gas field, 265 kilometres northwest of Darwin.
The Munupi elder says he was not consulted over the company's environmental plan and fears the project could damage his people's sea-country.
The offshore gas project would cut into the migration path of the vulnerable olive ridley sea turtle, marine ranger James De Santis said in his evidence on Tuesday morning.
Mr De Santis, who has monitored the turtles for years, described to the Darwin courtroom how the species swim out in the Timor Sea and return to the Tiwi Islands to lay.
"There's too much to lose out there if anything goes wrong," he said.
"It will be a great big loss for myself, my children and my grandchildren, right through generations of Tiwi people."
The drilling would disrupt the sea floor and a chemical spill would have a devastating effect on the thriving marine ecosystem, Mr De Santis said.
"If there is a spill, small fish can't break through so they die," he told the court.
"The big fish and birds will eat them and then be contaminated. It's a big cycle and everything ends up dead."
Mr De Santis did not recall having any contact with Santos about the drilling process.
Santos, which is Australia's second-largest independent gas producer, has previously said it would "vigorously defend" the Barossa project, and that it had all necessary approvals following consultation with stakeholders.
The $US3.6 billion ($A5.2 billion) offshore natural gas development is expected to create up to 600 jobs and pipe gas 280km to the Darwin LNG facility, with first production expected in 2025.
The company says the project, which it purchased from ConocoPhillips in 2020, is 43 per cent complete and on schedule. The drilling that is the subject of the court case started in July.
The Federal Court judicial review of the National Offshore Petroleum Safety and Environmental Management Authority's decision started on Monday at a bush court on Melville Island.
The historic hearing - the first of its kind - started with a smoking ceremony to welcome Justice Mordecai Bromberg and a dozen lawyers.
Evidence was given from four Manupi people, who told Justice Bromberg they were connected to the land and sea by songlines and totems, and feared the Santos project could damage the environment, which would in turn impact their way of life and spiritual wellbeing.
Evidence was also given in the form of song and dance, with four Manupi men wearing traditional face paint and clothing performing in the middle of the court.
The hearing continues on Tuesday afternoon.
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