An academic analysis of Covid-19 case has revealed how much more effective alert level 3 and 4 were in New Zealand compared to the lockdown in Australia.
And Otago University Associate Professor Brian Cox, a medically-trained epidemiologist and specialist in public health, believes New Zealand now has a chance to eradicate the deadly virus.
"If we were not to now achieve eradication it would be both a public health and economic failure," he told the Weekend Herald.
Cox had previously compared the number of confirmed Covid-19 cases per capita in New Zealand and Australia over a period that covered the 33 days of alert level 4.
His updated analysis now includes alert level 3.
"Overall, our confirmed per capita Covid rates have been 13 per cent lower than Australia.
"They have been 43 per cent lower from when we went into level 4 lockdown until the first day of level 2."
At the start of level 4, there were 53.6 confirmed cases per million people in New Zealand, rising to 229.7 at the start of level 3, and 234.8 on day one of level 2 - a seven-week period.
The rate in Australia at the start of its respective lockdown was 43.9 confirmed cases per million people, rising to 280.1 on May 15, eight weeks later, when most states had moved into stage one of Australian PM Scott Morrison's economic reopening (similar to New Zealand's level 2).
Cox said the data showed how Covid-19 spread more rapidly in New Zealand at an earlier stage, but the lockdown had fought back the virus more effectively than across the Tasman.
The data showing the success of NZ's lockdown over Australia's
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"We have achieved greater control of the epidemic than Australia. The lockdown in New Zealand has achieved a greater reduction in daily confirmed detection rates of Covid-19 than in Australia," he said.
"This is largely due to the strict lockdown we undertook so that action could focus on the clusters that formed the epidemic.
"Although Australia is two weeks ahead of us in the epidemic, they have had twice the rate of Covid-19 over the past week [up to May 15]."
There has been persistent questioning of whether New Zealand could have had more lenient lockdown rules , as Australia has appeared to have achieved similar public health outcomes while allowing hairdressers, retailers, construction and manufacturing to continue operating.
Ardern has pushed back strongly on any suggestion that the lockdown measures in New Zealand were too severe.
Epidemiologist Sir David Skegg has previously suggested that Australia's lower infection rate at the start of its epidemic curve was due to having better public health resources, including contact tracing capacity.
He has told the Epidemic Response Committee that successive governments in New Zealand had chronically underfunded the public health units here.
Cox's analysis suggests that New Zealand needed to implement a more stringent lockdown because the virus was threatening to spiral out of control.
He compared the two countries from the first day of each country's epidemic - or the day each country reached four confirmed cases per New Zealand's 4.9 million people.
Four separate cases are considered suggestive of community spread, he said.
That starting point was March 7 in New Zealand and February 22 in Australia.
Cox tracked the three-day rolling average in the daily confirmed case rate to iron out daily spikes.
New Zealand's level 4 lockdown started 19 days into the epidemic, while Australia took four to five weeks to implement different restrictions in different states, starting in earnest on March 24 and escalating within a week to a two-person limit on gatherings.
The success of both countries has opened the prospect of a transtasman Covid-free bubble , where citizens of both countries might be able to travel freely across the border without potentially importing Covid-19.
Ardern has said that while such a bubble would breathe life into trans-Tasman tourism and businesses, it would be more than "weeks" away.
Eradication possible, not just elimination
New Zealand is on course to eliminate Covid-19 , though what that means exactly has not been specifically defined.
Prime Minister Jacinda Ardern and health chief Ashley Bloomfield have said that it doesn't mean no cases at all, or eradication.
But Cox said that eradication was now possible, given the string of days with zero or near zero new cases.
"The talk was about reducing the incidence of Covid-19 to such a low level that we could achieve total control of this pandemic. However, the results of the last week clearly indicate that total eradication is within our grasp.
"Eradication would have considerable long-term economic advantages for trade and commerce."
Cox said that an ongoing concern was the St Margaret's cluster, which has led to seven Waitematā DHB nurses becoming infected.
"Infection among health care workers can, as was found in Italy, quickly propagate the epidemic by creating large clusters. Closing this cluster will be vital to our success."
The DHB has apologised to the staff who were caring for St Margaret's patients at Waitakere Hospital.
A review found that nurses were forced to change their protective wear up to eight times a shift, one of a suite of issues that may have led to staff catching the virus.
He added that the Health Ministry's data showed that overseas arrivals had an infection rate about seven times higher than the national average, while it is about 10 times higher for Air NZ staff.
"Regular testing of Air NZ staff is likely to be needed for some time and international staff are likely to need to be separated from domestic flights for some time."
Air NZ have been testing air crew and airport staff for three weeks. All results have been negative, a spokesperson for Air NZ said.
Thirty staff had previously tested positive for Covid-19, and they have all now recovered, the spokesperson said.