* Australia warns virus outbreak will take weeks to tame

* Mining sub-index hits highest level since August 2011

* "Big four" banks down over 1%

July 20 (Reuters) - Australian shares slipped on Monday after authorities warned that a surge in COVID-19 cases in the country's second most populous state could take weeks to tame.

The S&P/ASX 200 index fell 0.3% to 6,018.5 by 1251 GMT, dragged by energy and financials stocks, after a 0.4% gain last week.

Australia's acting chief medical officer said the coronavirus outbreak in Victoria state could take weeks to subside despite a lockdown and orders to wear masks in Melbourne.

The respiratory disease caused by the novel coronavirus has flared up in Victoria in July, mainly in Melbourne, with a daily record of 438 new cases detected on Friday.

The energy sub-index led the fall on the benchmark stock index with a 2% drop as oil prices dipped. Woodside Petroleum, Santos and Oil Search slipped between 2.1% and 3.2%.

The financial sub-index slipped 1.3% and was headed for its worst day in nearly two weeks, with all the "big four" banks down more than 1%.

However, South32, the world's largest manganese ore producer, said it would book a near $100 million impairment charge on its manganese alloy smelters, sending its shares down over 3% in early trade.

The metals and mining sub-index rose 1.3% to its highest level since August 2011.

Resolute Mining was the top percentage gainer on both the sub-index and the benchmark with a 6% jump, after a new plan for its Mako gold mine extended the mine life and total gold production estimates.

Shares of Rio Tinto climbed as much as 0.8%, still riding on robust second-quarter iron ore shipments reported on Friday.

In New Zealand, the benchmark S&P/NZX 50 index rose 0.14% to 11,599.9 by 1251 GMT, with Chorus Ltd adding 2.6%, while Contact Energy Ltd gained 2%. (Reporting by Arundhati Dutta in Bengaluru; Editing by Subhranshu Sahu)