South Australia will throw open its borders to millions of Australians from midnight on Tuesday – more than a month ahead of its original planned date.

Premier Steven Marshall said people from Western Australia, the Northern Territory and Tasmania will be able to enter the state without being required to go into 14 days quarantine from 11.59pm (local time).

Anyone who has recently arrived in SA from those states and is currently in isolation will also be able to move into the wider community.

Visitors from other states will have to wait until July 20 for restrictions to lift. That was the date Mr Marshall named last week for reopening the SA border to interstate travellers.

“We’ve received our advice and it’s pretty clear there’s no reason for us to be unnecessarily detaining citizens from Western Australia, the Northern Territory or Tasmania,” Mr Marshall said.

“So, effective from midnight tonight, there will be no requirement for 14 days of self-isolation.”

He said a decision on the Queensland border was also being considered. Queensland had no new virus cases on Tuesday, and has only five active infections.

“We didn’t make a decision on Queensland today, but it’s likely we will again meet later this week or early next week and we can again consider whether we can remove that state border with Queensland,” he said.

“We are still concerned about NSW and Victoria.”

• Businesses behind ‘travel triangle’ push for SA, WA and NT

WA Premier Mark McGowan hastily hosed down talk of a potential travel bubble with SA and the NT, saying his government had received Commonwealth advice that such a move would breach the constitution.

“It’s unconstitutional to pick and choose between the states, so we’d be breaking the law were we to try and do an arrangement with South Australia or the Northern Territory,” he said in Perth.

Strathmore Primary School has been closed after a student’s COVID confirmation. Photo: Google Maps

Victoria’s three-day spike

Victoria recorded nine new coronavirus cases on Tuesday – coming after 12 on Monday and nine on Sunday.

One was in an extended family cluster that now covers five households and 12 infections.

A third school was also closed in Melbourne a positive COVID-19 test in a grade five student.

Strathmore Primary School, in Melbourne’s north-west, followed Pakenham Springs Primary School, in the city’s south-east, and St Dominic’s in suburban Broadmeadows, which were shut for deep cleaning and contact tracing on Monday after pupils fell ill.

Victorian Health Minister Jenny Mikakos said parents at Strathmore were notified late on Monday of the positive test.

“The school is closed today and will be subject to a deep cleaning, and obviously the contact tracing will commence immediately to identify the staff and students who might be impacted by this particular case,” she said.

Despite the rash of school closures, chief health officer Brett Sutton said there was no cause for alarm.

“What’s reassuring for me, and I think for everyone, and it’s been the global picture is that when we look at the classroom contacts of the kids … they don’t pick up illness,” he told Melbourne radio 3AW.

“They’re very unlikely to pick it up from the kids in the classroom.”

Two more of Tuesday’s cases were in an emerging cluster linked to a patient at Monash Health. That outbreak has six confirmed infections.

Elsewhere, NSW reported three new cases on Tuesday morning. All were in people in hotel quarantine.

Green dots will guide the way on NSW public transport. Photo: AAP

NSW’s ‘world-leading’ public transport

The NSW government will double passenger capacity on public transport from July 1, representing another easing of virus restrictions as a result of low community transmissions.

Train, tram, bus and ferry passengers will, however, still have to maintain social-distancing between other commuters and be required to sit in certain seats, signified by green dots.

“I don’t know anywhere else in the world which has those indicators for the customers,” NSW Premier Gladys Berejiklian said.

“This is world-leading – I am incredibly proud of what NSW has done.”

The announcement came as NSW reported just three new coronavirus cases on Tuesday – all in people in hotel quarantine.

NSW Transport Minister Andrew Constance said the doubling of capacity would allow 68 people per carriage on a Waratah train, and 23 passengers on a two-door bus – up from 12.

On a freshwater ferry, 450 people will be permitted (up from 245), while 65 people will be allowed on a Metro train.

“This will give us capacity up to about 1.3 million people, particularly if people retime their day and reschedule the way in which they get around,” Mr Constance said.

He said increased cleaning across the network would continue.

“It’s really important that people continue to follow the guidelines, we still have marshals out there, we are continuing to clean trains on average about three or four times a day, buses three or four times a day,” he said.

-with agencies