Seven Australian cities are now in lockdown as authorities scramble to prevent the spread of the highly contagious Delta coronavirus variant.

Officials reported a slight case rise on Wednesday, to more than 200 cases.

Nearly half the population - more than 12 million people - are under stay-at-home orders in Sydney, Brisbane, Perth, Darwin, Townsville and the Gold Coast.

On Wednesday, the outback town of Alice Springs also entered a snap lockdown after cases emerged in South Australia.

Authorities fear the virus could now spread to nearby Aboriginal communities which are already vulnerable.

Across the country on Wednesday, state leaders said they were facing a "pressure cooker situation" as new cases emerged.

Many leaders have urged faster vaccinations as just 5% of the population is fully vaccinated.

But messaging around the country's main vaccine, the AstraZeneca jab, has been contradictory.

Vaccine contradictions confuse public

If you woke up in Australia today, you'd be forgiven for being confused about vaccinations. There's been the slow rollout, the lack of supply, and vaccine hesitancy. Now, add mixed messaging from the leadership to this list and you've got a perfect storm.

In a big U-turn on Monday Prime Minister Scott Morrison announced that anyone under 40 who wants the AstraZeneca vaccine could have it after talking to their GP.

That message was quickly rejected by the Australian Medical Association's president, who said it took him by surprise and went against expert advice. The Australian Technical Advisory Group on Immunisation (ATAGI) recommends AstraZeneca for over 60s.

State premiers then also accused the PM of wrong guidance, while criticising the shortage of the Pfizer alternative.

Delta has breached Australia's defences faster than anticipated. It's underlined how slow and at times shambolic the vaccine rollout has been.

Australia remains in an enviable position globally, with an overall low number of Covid-19 cases and deaths.

But the next few weeks will be crucial - with the country's Covid success now hanging in the balance.

The Delta variant has been found in five of eight states and territories, just a fortnight after it emerged in Sydney.

Australia had prevented wider Covid transmission for the past year through stringent measures. These included closed borders, hotel quarantine and aggressive contact tracing systems.

But leaks from quarantine have highlighted gaps in the country's defences.

It has also exposed the vulnerability of a largely unvaccinated population. Mr Morrison has been widely criticised for the vaccine rollout's failures.

Calls to tighten borders

On Wednesday, Queensland Premier Annastacia Palaszczuk also called for a crackdown on international arrivals.

She said the state's concerning new Delta cases had come from a business traveller from Indonesia who had infected a hospital receptionist. Three cities in the state - Brisbane, Townsville and the Gold Coast - entered a three-day lockdown on Tuesday.

"The person who brought the virus into Queensland was a regular traveller, not a vulnerable Australian returning home... I honestly think we need a serious discussion about ensuring that people are vaccinated coming into this country," said Ms Palaszczuk.

"We have got to minimise the risk. We are at a pressure cooker moment at the moment. Right across Australia."

Western Australia and Victoria have also called for a reassessment of arrivals allowed into the country.

New South Wales recorded 22 new cases on Wednesday, taking its cluster to about 170 cases. Its capital, Sydney, and surrounding regions remain in lockdown until 9 July.