Health authorities are hunting for the source of the four new cases, all of which are close family contacts of each other, in the Whittlesea area north of Melbourne.

The new cases bring a new sense of urgency around the slow rate of vaccination in some areas of Australia, where people under 50 are able to volunteer to get the AstraZeneca vaccine in otherwise empty vaccination centres.

In South Australia, the vaccine program will be extended to anyone above the age of 16 in regional areas, the first state to do so in a bid to keep up momentum in the rollout.

"We will always be at risk until we have high vaccination coverage," the Victorian Chief Health Officer Brett Sutton warned on Monday.

"You never know what is around the corner and, again, I flagged several countries that have gone from zero community transmission to literally hundreds and, for some countries, thousands of cases per day."

The federal opposition has attacked the government for giving mixed messages to people aged over 50 about whether or not they could wait until later in the year to receive the Pfizer or Moderna jab, over concerns about blood clotting related to the AstraZeneca vaccine.

Health Minister Greg Hunt said in question time Labor was misquoting him on when over-50s should get vaccinated.

"The message is very clear: if you are not vaccinated and you catch Covid, you could die. That's the message to all Australians. There could not be a clearer message," Mr Hunt said.

Commonwealth Chief Medical Officer Paul Kelly said the cases were a reminder of how important vaccination was.

"We are approaching winter very quickly," he said.

"We've been in a very good position this year in terms of acquired cases in Australia but that can change very quickly."

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Despite the please from government health officials, there are fears Australians aren't rushing to get vaccinated, out of hesitancy around side-effects and a lack of urgency around risk.

Asked on Monday if Australia should consider novelty incentives used in other countries, like house lotteries, cash lotteries and other discounts, Professor Kelly said any measure should be considered.

"I think all of these things are potentially on the table," he said.

"At the moment we know there is some hesitancy, particularly in the 50-69 year age group. We are rolling out extremely well in the over 70s. So there is something we need to consider there. And, you know, we can certainly look at that."

Despite that, Professor Kelly said protecting against illness and death was the number one incentive for getting vaccinated.

"Once an outbreak is here, if it comes, it will be very difficult for us to rollout masses and masses of vaccines quickly. So the more we can get vaccinated now, the better the situation will be when that day comes."

ACT Health Minister Rachel Stephen-Smith agreed with Professor Kelly on the main incentive for vaccination being protection against the virus.

"Any idea to encourage uptake for the COVID-19 vaccine should be explored. However, we would need to see much more detail about this proposal before agreeing to it," she said.

Ms Stephen-Smith said the ACT's circumstances were different to those of South Australia and there wasn't the same need to balance factors like regional areas with small population sizes.

"This means we do not face some of the same logistical challenges as other states and territories, including how to safely manage vaccine delivery across large geographical areas or ensuring enough staffing for regional vaccine clinics," she said.

"We are currently vaccinating people eligible under phase 1a, 1b and any person over 50 years old in the ACT. We will open bookings to those in later phases as the program continues to roll out."

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