SYDNEY, July 20 — A surge in Covid-19 cases in Australia's second-biggest city could take weeks to subside despite a lockdown and orders to wear masks, Australia's acting chief medical officer said today as the country braces for a second wave of infection.

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

Victoria's government has ordered about five million people into a partial lockdown for six weeks and told residents around Melbourne to cover their faces if they have to leave their homes.

Australia's Acting Chief Medical Officer Paul Kelly said it would take "weeks" to slow the outbreak to levels seen as recently as June, when Victoria and the rest of Australia reported single or double-digit daily infections.

"We have learned over time that the time between introducing a measure and seeing its effect is at least two weeks and sometimes longer than that,” Kelly told Australian Broadcasting Corporation radio.

Australia has recorded about 11,800 coronavirus cases, a fraction of what has been seen in other countries, and the disease has been effectively eliminated from most states.

But the Victorian outbreak and rising daily cases in neighbouring New South Wales, the country's most-populous state, are stoking fears of a national second wave.

NSW reported 18 new infections on Sunday, its highest in three months. The transmission rate in the state is higher than in Victoria, despite social distancing restrictions being tightened.

NSW authorities have been unable to trace some of the clusters and state authorities have urged people to avoid unnecessary travel and public transport.

Should NSW be forced to implement new restrictions, it would be a hammer blow to Australia's hopes for a quick economic recovery. Already, Australia is facing its first recession in nearly three decades, with unemployment at a 22-year high. — Reuters