Authorities in New South Wales are searching for 70 doctors who attended a radiology conference with two people affected by Covid-19 and assessing more than a dozen children who visited a Sydney nursing home where four people have been diagnosed with the virus.

“I think at this point it is fair to say that we do have an evolution happening in the spread of this virus,” New South Wales health minister Brad Hazzard said.

“We do know that containment is likely to be an unlikely outcome, so we need to work constructively with the community to continue our effort to emphasise to the community that [controlling the virus] is a partnership … Our community has to be doing everything they can do to support us in defeating what has really become a war in defeating this coronavirus.”

Four people at the Dorothy Henderson Lodge aged care facility in northern Sydney – a 95-year-old woman who has since died, a 82-year-old man, a 70-year-old man and an aged care worker in her 50s – have so far tested positive to Covid-19.

The 70-year-old man resided in a different area of the lodge and was cared for by different staff than the other affected residents, which Hazzard said suggested infection control protocols had not contained the spread.

The aged care centre has since been isolated.

NSW chief health officer Dr Kerry Chant said about 17 children from the Banksia Cottage childcare centre at Macquarie University visited the aged care home on 24 February, on a day when the infected aged care worker was not working. Health authorities are meeting with parents and children on Thursday night to conduct assessments.

Hazzard said that as a precaution, childcare centres had been advised to suspend their regular visits to aged care homes until the risk of children being “super-spreaders” of the disease had been determined.

He said that authorities had contacted 77 doctors who attended a radiology seminar on 18 February, which is being investigated as the possible source of infection for a 53-year-old doctor at Ryde hospital who tested positive to the virus this week. Another doctor at that seminar, a registrar in her 30s from Liverpool hospital, was diagnosed with Covid-19 this week.

Neither doctor had travelled overseas or had contact with known coronavirus patients.

Hazzard said the process of tracing back possible contact between infected people was “a bit like a police investigation in a sense, trying to track who is coming into contact with who and what possible associations there may have been”.

Meanwhile, a tourist in Darwin in Australia’s Northern Territory has tested positive for coronavirus. It is the first confirmed case in the territory, which contains hundreds of remote communities.

On Thursday NT Health said a 52-year-old man carrying the virus was in isolation in Royal Darwin hospital.

The man recently arrived in Darwin from Sydney and has had limited contact with the local community, NT Health said. The department was undertaking contact tracing and would reach out to those who may have been in contact with the man.

New South Wales confirmed overnight that the virus had spread to six new people in the state, bringing the number of cases to 22.

The death of the woman in aged care is Australia’s second recorded coronavirus death, after a 78-year-old who had been evacuated from the Diamond Princess died in hospital in Perth.

There have now been 52 confirmed cases of the virus in Australia since the outbreak began.

The virus continues to spread in NSW, where the majority of cases have been confirmed.

On Thursday prime minister Scott Morrison announced upgraded travel advice for South Korea, recommending people reconsider travel. Travel bans remain in place for China and Iran and citizens and permanent residents returning from those countries are required to self-isolate for 14 days.

Besides the aged-care resident, health authorities said a female doctor at Liverpool hospital, a woman in her 30s from the northern beaches, a male from Cronulla in his 50s and a woman in her 60s who is believed to have returned from the Philippines had all tested positive.

“The female doctor who was diagnosed on March 4 had no history of overseas travel,” Chant said.

“We are immediately establishing which staff and patients may need to self-isolate and be tested for Covid-19 should they be unwell.”

The woman in her 60s arrived back in Australia on 3 March and is believed to have returned from the Philippines.

“Her travel details are being obtained and will be disclosed if she posed a risk to any other passengers on her flight,” Chant said.

The aged care worker became ill at work on 24 February. She had not travelled overseas, and is the second person to have acquired the virus in the community and cannot be traced back to a source.

Both people worked with vulnerable people, including hospital patients and the elderly.

“It is concerning when we have somebody present with coronavirus and we can’t track the source,” Hazzard said.

“So that raises the question, how did she end up with coronavirus?” Hazzard said. The doctor had been in close contact with eight patients.

People are thought to be most contagious when they are most sick, so out of caution health authorities trace back an infected person’s contacts to the day before they fell ill. As a result, 11 of the health worker’s patients had been placed in isolation.

In Victoria on Wednesday, a man who has returned from Iran was confirmed as the 10th case of coronavirus in that state. Queensland now has 11 confirmed cases, including a 26-year-old man who returned from Iran on 26 February.

But on Thursday health authorities in Queensland said the housemate of a 20-year-old international student who contracted the virus was not also infected.

Queensland’s chief health officer, Dr Jeannette Young, said on Wednesday the man had felt unwell before the flight so authorities were tracking down people who had sat within two rows of him to recommend they begin 14 days of self-isolation. They have already identified and isolated nine people who had close contact with him.

The state’s health minister, Steven Miles, said it was unnecessary for people to stockpile supplies against the small chance that they would be asked to go into home isolation.