The reason for this can be summed up in two words, health experts say: community transmission.

In March, when the first round of lockdown restrictions were brought in nationwide, Australia was seeing a couple of hundred new cases each day – not much more than the number of daily cases coming through in Melbourne and surrounds over the past few days.

But a key difference this time is that these new daily cases are mostly from people who have caught the virus here in Australia, says Katie Allen, Federal Liberal Member for Higgins, paediatrician and former medical researcher.

“It’s not just the fact that we’re getting a second surge, it’s that we’re getting a second surge of a different nature,” she told RN Breakfast.

“Last time with the first surge, 90 per cent of cases were from overseas and they were being contained and quarantined, and the remaining 10 per cent were being tracked and traced.

In contrast, this time the vast majority of new cases are people who have caught the virus here in Australia, with many of whom it’s unclear exactly where they picked it up.

With the virus now spreading locally, it’s a much bigger job to keep a handle on the spread, says Dr Hassan Vally, an epidemiologist at La Trobe University.

“We are at a point where, unless we have drastic action, things could spiral out of control,” Vally says.

“We’re probably fairly close to the limit of the public health workforce to respond to cases and do the contact tracing and do everything else that needs to be done. We don’t want these 200 cases to go to 2000 cases.”

Part of the difficulty with responding to the virus is the lag time between when someone is infected and when they start showing symptoms, Vally says.

“What we are seeing now is five to 10 days old in terms of when people got infected, and of course interventions implemented now will take five to 10 days to see the effect of.”

There has been some level of community transmission in Australia since the beginning of the pandemic, but so far, isolating cases and contact tracing has allowed health departments to quickly contain outbreaks.

With triple-digit daily increases in Melbourne over the past few days, tracking and tracing individual cases was becoming untenable, leading to the additional Government action of imposing the new lockdown.

Lockdowns are also particularly appropriate for controlling COVID-19 because such a significant proportion of people who have it are asymptomatic, says health broadcaster Dr Norman Swan.

“That’s what a lockdown helps to control, because if you’re focusing just on symptoms, you’re missing people who are either asymptomatic or presymptomatic,” Swan said on the ABC’s Coronacast.

“It’s younger people who are getting infected and they tend to have milder symptoms, but yet are spreading it.”

How do you tackle community transmission?

The Victorian Government’s action so far is on the right track, Vally says.

Isolating cases, quarantining potential cases, lots of testing and detecting as many cases as possible are all essential to keeping a lid on transmission. Locking down the suburbs at highest risk reduces the number of human interactions, further limiting the virus’s ability to spread.

But there are responsibilities on individuals too.

Vally said people should be following the spirit of the tougher restrictions rather than pushing the limits of the rules.

“It’s up to people to be really earnest and very thorough in the way that they maintain physical distance, wash their hands and keep their hand hygiene going, make sure that they limit their exposure to people outside the house,” he said.

“Maybe, if they’re going to be in situations where they think they’re going to come into contact with people, now is the time for people to start to consider wearing a mask.”

This is a heavy blow for Melbourne, and other parts of the country should take note, Vally says.

“A bad as this is, for the rest of Australia, this could be the wakeup call that everyone needed. It can turn around really quickly.”

Swan agrees.

“New South Wales is probably the state that’s most worrying because there is still virus there at a very low level. So people have just got to be careful. They’ve got to maintain physical distance.”

And while it feels like we’ve been doing this for months — because we have, to varying degrees — we’re still in this for the long haul.

“While we don’t have a vaccine, we’re in this delicate balance,” Vally says.

“We’re trying to suppress transmission by all of these public health interventions but, of course, we’re experimenting with allowing people to have more freedoms. We’re always at risk of tipping in the direction where the virus starts to take hold.

“We’re going to have to navigate this balance at least for another six months, maybe six to 12 months, maybe even longer. This is the new normal.”

-ABC/Tegan Taylor