Torres Strait Islanders could be forced to leave their homes within the next 30 years, if urgent action is not taken on climate change.
This would mean a loss of country, sacred sites and culture, the Federal Court has been told.
Uncle Paul Kabai, from the low-lying Saibai Island, said he's scared of having to leave his country, in an affidavit read to the court in Cairns on Monday.
"My country would disappear," he said.
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READ NOW"I would lose everything; my country, my culture, my stories and my identity. Without Saibai I do not know who I am."
Mr Kabai and Uncle Pabai Pabai, are leading the first climate class action brought by Australia's First Nations people.
The Torres Strait Island elders launched the court action in 2021, faced with rising sea levels and fearing their communities could become Australia's first climate refugees.
They are arguing the Commonwealth owes a duty of care to Torres Strait Islanders to take reasonable steps to protect them from the harms caused by climate change.
Large parts of the islands could be uninhabitable by 2050 years, forcing Torres Strait Islanders to leave their ancestral homelands, lead counsel for Mr Kabai and Mr Pabai, Fiona McLeod, said in closing submissions.
"This case concerns an Incontrovertible truth ... our First People in the Torres Strait will be brutally impacted by climate change and they will have and will continue to suffer devastating losses," Ms McLeod said.
"Potentially in the lifetimes of these two elders Pabai Pabai and Paul Kabai ... they will face the losses of their precious lands and waters, their ancient and proud traditions, the mass extinction of species including totemic creatures, their ability to practice ceremony on country and the resting places and the remains of their ancestors."
The court held on-country hearings on Badu, Boigu and Saibai islands in 2023, while scientists and other expert witnesses gave evidence in Melbourne in November.
Aunty McRose Elu, who is also part of the class action, said she has seen her ancestral homelands on Saibai Island inundated with water over decades.
"As you go through the area when you fly over it's very frightening," Dr Elu told AAP.
"Now you can see more water than land but people live there."
Dr Elu said at the case is about the continued existence of Torres Strait Islander people and their homelands.
"What we're fighting for is survival," she said.
"We want to save our islands, we don't want our islands to go under the water.
"That's why it's important to me, we think about the young people who are yet to come ... I don't want them to see that they have no land and no island."
The Federal Court will sit in Cairns until Friday, with the Commonwealth's closing submissions expected to follow Ms McLeod's.