In short:
The protected waters around the remote Heard and McDonald Islands in the Southern Ocean are set to quadruple in size.
The federal government's plan to expand the marine park for the subantarctic waters would help protect vulnerable species.
What's next?
Marine conservationists want some of the proposed boundaries changed to ensure the most important areas receive the highest levels of protection.
The marine park that includes the subantarctic Heard and McDonald Islands is set for a massive expansion, with the federal government planning to protect an extra 300,000 square kilometres of ocean.
The expansion would quadruple the size of the current protected area, providing a haven for penguins, seals, whales and albatross, as well as protecting important breeding and feeding grounds for sea life and birdlife.
Heard and McDonald Islands are in the Southern Ocean, 4,000 kilometres south-west of the Australian mainland and 1,700 kilometres north of Antarctica.
The islands, which are among the least disturbed by human activity in the world, are considered to be one of the world's most pristine areas, and are listed as a UNESCO World Heritage site.
Currently, 17 per cent of the waters around Heard and McDonald Islands have been designated a sanctuary zone that is closed to the public.
The proposed expansion would see national park and habitat protection zones established in much of the remaining waters in Australia's exclusive economic zone around the islands.
It would mean 90 per cent of the surrounding waters would be protected to some extent.
"This is a unique and extraordinary part of our planet; we have to do everything we can to protect it," federal Environment Minister Tanya Plibersek said.
"It is untouched by humans. There are no weeds, there are no introduced species and it is home to whole range of threatened species that we want to better protect, including whales, seals [and] albatross."
Key areas still unprotected, say marine ecologists
The proposal has been welcomed by marine conservationists, but those same conservationists remain concerned that the most sensitive locations won't be adequately protected.
Dr Andrew Constable, from the Centre for Marine Socioecology at the University of Tasmania, is an expert on the ecosystems at Heard and McDonald Islands.
"At the moment, new areas for sanctuary protection are not in areas that we would recommend [as the] highest priority," Dr Constable said.
He would like to see an area of shallow water that is important to fur seals made a sanctuary zone, along with underwater ridges and canyons that are vital for penguin breeding and sea floor organisms.
Richard Leck leads WWF-Australia's campaigns to protect the ocean, and believes moving some of the sanctuary zone's proposed boundaries could make a significant difference.
"There are some key habitats that are important to penguins — particularly the macaroni penguin — that remain unprotected under the new proposal," he said.
"Those areas won't overly impact on [the] existing fishing industry, and it would be great to see that proposal revised and those important species protected."
Both Dr Constable and Mr Leck were hoping to see the high-level protection put in place last year around almost all of Macquarie Island (another remote subantarctic territory) replicated at Heard and McDonald Islands.
"We want to see similar a level of ambition as the government has already implemented in our other subantarctic islands," Mr Leck said.
"Australia is incredibly fortunate to be a custodian of this incredible place, and we really should be giving it the protection it deserves."
'Sustainable' commercial fishing to continue in limited area
The proposal would allow current commercial fishing to continue, with mackerel icefish and Patagonian toothfish fishery operations considered sustainable.
Ms Plibersek said the existing fisheries could "co-exist" with the expansion of protected areas, noting they help deter illegal fishing.
"At the moment, we have a very co-operative arrangement with the existing fisheries companies that operate in the area … they are very conscious of any attempts to do illegal fishing in the area." she said.
Marine ecologists are satisfied the current fishing operations are sustainable, but believe some of the sanctuary boundaries could be changed to better protect some sea floor areas.
There will be public consultation on the proposed expansion and the boundaries until September 5.
Remote islands 'on the frontier of climate change'
The islands' remoteness is key to their ability to provide a haven for sea life and birdlife.
But as remote as they are, subantarctic islands are "on the frontier of climate change", according to Dr Constable.
"There are species that are living in the vicinity of Heard Island, in our territory, that are vulnerable to the effects of climate change," he said.
"As the ocean warms, these cold-adapted species may be lost — and we need to do everything we can to provide resilience to those species."
The government will also fund a research expedition to Heard and McDonald Islands in 2025 or 2026, at a cost of $17.6 million.