Victoria has recorded 466 new Covid-19 cases since yesterday's update and 12 deaths.

One death was a man in his 30s, two men in their 70s, two men and three women in their 80s and four women in their 90s.

"We send our best wishes and condolences and sympathies to those families and we acknowledge this will be a very difficult time for them," Premier Daniel Andrews said.

"They are in our thoughts and prayers."

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Authorities are "assuming" there are more Covid-19 cases than detected in Victoria, and Premier Daniel Andrews says that is why restrictions are necessary.

"I understand there are some communities who [have a] sense of frustration at seemingly being free of Covid-19 but having these restrictions. I can appreciate that, but the number of mystery cases we have across the state and some in regional Victoria, we have to assume there is more virus, more transmission, more cases out there than the data tells us," he said.

"That abundance of caution is critically important to get to the other side of this as a whole state and not necessarily have a situation where there might be reductions in Melbourne but see increased numbers in regional big courier.

"We don't want to have that happen."

Meanwhile, Queensland closed its border to NSW residents overnight, and anyone attempting to cross the border today was warned to expect big delays.

The restrictions began at 1am today. All NSW visitors are denied entry to Queensland except for rare exemptions.

Queenslanders returning to the state will also have to pay for 14 days in mandatory hotel quarantine.

Queensland Police Chief Superintendent Mark Wheeler said people were waiting more than 90 minutes to get across the border, and at least 142 people had been refused entry this morning.

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"The 142 people who were turned around were disappointed, and the 18 Queenslanders no doubt were extremely disappointed, because there's going be considerable cost for them," Wheeler said this morning.

"Not only when they fly back home, they also then need to go into quarantine at their own expense. So, it's a very costly exercise. But there was significant warning put out during the week. So, it's unfortunate, but this is all about stopping Covid-19 coming into Queensland."

Wheeler said the delays and long lines were expected.

"We knew it was going to be busy. We knew the line-ups were going to be incredibly long," he said.

"Every time there's been another iteration of border restrictions, we have been prepared for that, and we had additional staff working."

Wheeler said the people who were turned around either came from NSW, the ACT or Victoria and didn't have an appropriate pass to get across the border.

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Victorian protective service officers on patrol walk past a sign urging people to stay home during the Melbourne lockdown. Photo / AP
Victorian protective service officers on patrol walk past a sign urging people to stay home during the Melbourne lockdown. Photo / AP

People aged between 20 and 29 account for the largest number of Covid-19 cases and the highest rate of infection in Australia, according to the Department of Health.

Professor Jodie McVernon, from the Doherty Institute, told the Today show there are a number of reasons for that.

"A lot of the essential workers out there making sure that we can buy our groceries or in healthcare settings are younger people," McVernon said.

"So they are required to mix for work."

She also explained a lot of the transmissions occur in families and households where people are not required to wear a mask.

"We need to be able to touch and be close to those people who we live with, if we're fortunate enough to do that," she said.

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"So we are seeing lots of spread in families and that will bring the average age down because we see children and young people infected in that environment as well."

Peter Collignon, Professor of Infectious Diseases at the Australian National University, said the challenge was that while young people were more likely to be infected, they were less likely to get very sick, leading to complacency.

"That differential in death [rates] and also your chance of going to hospital probably also affects your view that 'I'm indestructible and this won't affect me'," he told SBS News.

"Now it is a problem, because even if your risk of death is less than one in 1000, if 100,000 people get infected, that's still 100 deaths that would be otherwise avoidable.

"But I think it's all those factors, the fact that they are the most affected often socially and economically but the least affected medically, means that they may be less likely to comply with the basic things we need to do."