Australia's state of Victoria recorded its worst day of Covid-19 fatalities yesterday, but experts believe the current lockdown is bringing the outbreak under control.

Ten days after Victoria's government imposed a strict lockdown on Melbourne, the country's second-largest city, the authorities yesterday said 410 new cases had been confirmed in the state and a further 21 deaths. The current six-week lockdown began on Aug 2 and includes a curfew from 8pm to 5am and the closure of schools and all non-essential shops. This was expected to lead to reduced case numbers by the middle of this week.

Victoria's Premier Daniel Andrews said the current lockdown was working, despite case numbers increasing from 331 on Tuesday. There were 322 new cases on Monday and 394 on Sunday.

"If you look at the average over the last seven days, we are seeing the line come down," he told reporters. "It's not exact because it's all depending on literally hundreds of millions of individual choices and decisions that each of us makes every single day."

The recent strict lockdown was imposed after previous measures appeared to be failing to reduce case numbers. The authorities said sick people continued to go to work - particularly casual workers who were not entitled to sick leave - and people were flouting requirements to stay at home.

Associate Professor Ben Phillips, an ecologist at the University of Melbourne, said he believed case numbers were likely to drop fast over the coming days. He said the increase yesterday may be due to variations in daily numbers caused by factors such as the timing of processing test samples. In addition, he said, Covid-19 case numbers tend to decline much more slowly after lockdowns than they go up during outbreaks.

"Case numbers in Victoria are dropping and should continue to drop quite fast," he told The Straits Times. "The previous lockdown was having an effect, but it was slower than everyone would have liked. It is much harder for the virus to propagate now."

He added that the increase in number of deaths in recent days was occurring because fatality numbers lag behind new infections.

"On average, it takes 17 days between presenting an illness and getting sick enough to die. We will continue to see an unsettling number of deaths in the near future, but it should then continue to drop off."

As of yesterday, Australia had recorded 22,127 Covid-19 cases and 353 deaths. Outside Victoria, New South Wales, the most populous state, recorded 18 new cases yesterday. No other state or territory had any new cases.

Official data released by Victoria yesterday showed that the state has had 15,251 cases of Covid-19, including 7,880 active cases. In total, 3,610 of the cases have involved 20-to 29-year-olds, who have been the worst-hit age group.

Experts believe the higher rates of infection among younger people are due to their heightened risk of exposure. Younger people are more likely to gather in larger groups, attend workplaces or be involved in parental activities and shared child-minding that expose them to other families.

Victoria's government indicated yesterday that it may continue to require people to stay at home for several weeks even after the current measures end on Sept 13.

"I would love to be able to confirm for you what we're going to be facing in October," Mr Andrews said. "We can't know that."

Prof Phillips said the success of Australia's first lockdown in March and April may have prompted Victoria to respond too slowly to its second outbreak in late June.

"We had a very rapid improvement with the first lockdown, largely because we didn't have much community transmission," he said. "We were set up to think it was easier than it should be, which, on the second time, made us react slower than we should have."