SYDNEY, Nov 16 (Reuters) - Australia will spend A$1 billion ($726.3 million) underwriting construction of a vaccine manufacturing plant under a deal with a unit of biomedical giant CSL Ltd, guaranteeing supply of flu shots and antivenins, the federal government said on Monday.

Under the deal, CSL vaccine subsidiary Seqirus would spend A$800 million building the facility in the city of Melbourne, while the government would guarantee purchase of its vaccines for 10 years, local media reported.

The purchase agreement would begin when the new plant opens in 2026, in time for the expiry of a current supply agreement with CSL which runs until about 2025, media added.

"We're currently well placed, but we want to be even better placed because who knows what will come," Health Minister Greg Hunt told the Australian Broadcasting Corp.

"We'll have that national vaccine manufacturing capability as well as the long-term contracts for flu and for anti-venoms."

Hunt added the new CSL plant would be the largest vaccine manufacturing plant in the southern hemisphere.

The Australian government has already struck agreements with CSL to supply two potential vaccines for the new coronavirus which are being trialled by Britain's AstraZeneca Plc and Australia's University of Queensland.

($1 = 1.3768 Australian dollars) (Reporting by Byron Kaye)