Australia has increased its share of the seabed by 2.5 million sq km, after the UN agreed an extension.
Its continental shelf will be extended by five times the size of France, giving it rights to explore new areas for oil and gas.
Australia had worked for 15 years on the claim.
Some of the new areas lie beside Antarctic territories, but the resources minister said there would be no exploration of Antarctic waters.
"I am pleased to announce that Australia, the largest island in the world, has just been dramatically increased in size," said the resources minister, Martin Ferguson.
He said the new territory was 10 times the size of New Zealand and 20 times the size of the UK.
The minister said he could not put a figure on the potential oil and gas reserves contained in the areas.
"The truth of the matter is that they have been hardly explored," he said.
"This is potentially a bonanza. We have got unknown capacity up there."
Mr Ferguson said it was important that Australia got on with exploration, because it was not rich in oil.
Australia's continental shelf is being extended beyond the 200 nautical mile limit, in parts of the west, south and east, and also beside the Antarctic territories of Macquarie and Heard islands.
The claim was made under the UN Convention on the Law of the Sea.