The Ross Sea is considered the last remaining pristine ecosystem on the planet, earning it the label "The Last Ocean". It is home to penguins, Antarctic toothfish, whales and Weddell seals.
The protection zone is one of New Zealand's major foreign policy objectives, and it has taken six years of diplomatic wrangling to get all countries to agree to the proposal.
Commission decisions require a consensus, and proposals can fail if any single country objects. Previous attempts to reach agreement have been scuppered by opposition from fishing countries, mainly Russia and Ukraine.
McCully lobbied his Russian counterpart Sergei Lavrov on the issue during a recent trip to Moscow.
United States Secretary of State John Kerry - who has a strong interest in marine protected areas - also played a role in convincing Russia of the merits of the protected area.
"At a time when relations on so many fronts are difficult with the Russians, some co-operation and a constructive dialogue is very pleasing to us," McCully said.
The NZ-US proposal required some changes to secure the unanimous support of all member countries. The final agreement balanced marine protection, sustainable fishing and science interests, McCully said.
One of the concessions made was an agreement to allow more of the lucrative Antarctic toothfish species to be caught within a designated, fish-rich research zone.
Countries agreed to raise the proportion of the total allowable catch for the Ross Sea region which could be caught within the zone from 13 per cent to 15 per cent.
"Getting 25 countries to agree to a proposal like this was always going to be hard," McCully said.
"We obviously haven't got everything that we asked for. But we've got a big chunk of it."
The new sanctuary could mean New Zealand is required to increase its monitoring of the Antarctic region for illegal fishing. The Royal New Zealand Navy conducts annual patrols of the Southern Ocean.
McCully said any monitoring decisions would come at a later date.
