Leonardo DiCaprio urged a high-level Antarctic conservation meeting in Australia Tuesday to approve a sweeping Southern Ocean marine sanctuary to protect thousands of polar species.

The Hollywood star wrote an email to supporters of activist group Avaaz calling for a "people-powered surge" in favour of the sanctuary, which is headlining talks at the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).

The key theme of the 2012 CCAMLR summit, which officially opened in Hobart on Tuesday, is the establishment of a representative system of marine protected areas in the Southern Ocean as proposed by Australia, France and the EU.

The region is home to big populations of penguins, seals and whales found nowhere else on Earth, and also has unique seafloor features that nurture early links in the food chain, according to environmental groups.

DiCaprio said most countries backed a marine haven off East Antarctica which, if approved in the size being advocated by green groups, would be the world's largest ocean sanctuary sprawling over an area twice the size of India.

"But Russia, South Korea and a few others are threatening to vote it down so they can plunder these seas now that others have been fished to death," he said, advocating an online petition with more than 842,000 signatures.

"The whales and penguins can't speak for themselves, so it's up to us to defend them. Let's change negotiators' minds with a massive wave of public pressure."

Made up of 24 countries and the European Union, CCAMLR was established in 1982 with the goal of conserving marine life in the face of rising demands to exploit krill, a shrimp-like creature which is an important source of food for species in the Antarctic.

The Commission permits fishing provided it is carried out "in a sustainable manner and takes account of the effects of fishing on other components of the ecosystem".

DiCaprio said climate change was already reducing many creatures' habitats and industrial fishing would further reduce their chances of survival, urging against dilution or delay of the sanctuary proposals currently before CCAMLR.

"The future of the Southern Ocean is in our hands," said the star of the movie "Titanic", which is based on the ship which hit an iceberg and sank during its maiden voyage in 1912.

"Let's unleash a massive surge of global pressure and ensure governments don't put profits before our planet."