The lid on a shared rubbish bin.
That is how a person is thought to have become infected before boarding a charter flight from Christchurch to Auckland and infecting a man on that flight - both of whom having completed 14 days in managed isolation, with negative tests on day three and day 12.
Ministry of Health officials say the rubbish bin has been pinpointed as a "common factor" after extensive contract tracing to identify the source of the passengers' infection.
The rubbish bin was in a corridor on the floor of a Christchurch managed isolation facility, and people had to lift the lid to deposit rubbish.
Between September 5 and 8, according to CCTV footage, a person with Covid-19 (who tested positive on September 9 and likely caught Covid-19 on a flight from India) shared the bin with a person who ended up on the September 11 charter flight.
"While we cannot be certain, our hypothesis is that the virus may have been transmitted to a person ... via the surface of a rubbish bin," said director of public health Dr Caroline McElnay.
The person then sat behind a man on the charter flight whose positive test was reported on September 19.
Previously the man was thought to have had an unusually long incubation period, having tested positive three weeks after flying into New Zealand.
Now he is thought to have caught Covid-19 on the flight from the person behind him, who didn't have any symptoms but tested positive on September 23, following the testing of all close contacts on the charter flight.
McElnay said people in managed isolation had to put their own rubbish out in communal bins. There were signs telling them to use hand sanitiser before and after using the bin.
"This particular bin had a lid that required you to lift the lid."
All bins would now be touch-free, she said, while all rubbish must be securely sealed in plastic bags prior to being placed in the bins.
It was not dissimilar to the case at the Rydges Hotel, where an infected traveller is thought to have passed Covid-19 to a maintenance worker who pushed the same elevator lift button.
"What we're seeing more and more of is that surface contamination can be a cause of Covid transmission."
Timeline of Covid infections from flight from India
August 27: Flight arrives from India via Fiji with returnees who are transferred to managed isolation in Christchurch.
August 30: Two people test positive on day three testing and are transferred to quarantine. We think one was infected in India (first person), and subsequently infected the other on the international flight (second person).
September 10: Ministry of Health reports case from managed isolation in Crowne Plaza in Christchurch has tested positive on day 12. Case transferred into quarantine (third person). We think this person was also infected by the first person, on the international flight, and was incubating the virus at their day 3 test.
[Third person shares common rubbish bin in MIQ with fourth person].
September 11: Charter flight from Christchurch to Auckland. Fourth person and child sat behind fifth person.
September 19: Fifth person reported as testing positive for COVID-19.
September 20: Fifth person's two household contacts reported as testing positive.
September 23: Fourth person and child reported as testing positive. Household contact also tests positive.
No new cases
There are no new cases of Covid-19 in New Zealand, McElnay announced today.
That means the total number of cases in the country remains 1492.
No one is in hospital with the virus. It comes after one person had been held in isolation on a general ward at Middlemore Hospital for several days.
There are now 43 active cases, including 32 imported cases and 11 community cases.
Yesterday 4403 tests were processed.
Following revelations yesterday that 10 people had Covid-19 on the same flight from India, McElnay issued a plea to those wanting to fly into New Zealand.
"In the 14 days prior to departure, people about to return to New Zealand should consider avoiding going to parties, social gatherings, or crowded places, avoiding contact with people who have Covid-19 or who have been in contact with cases, and staying home as much as possible to limit exposure to other people.
"Doing these things will help reduce the risk of being exposed to Covid-19, then bringing it home with you. People should also be aware of symptoms and get a test if any develop."
This morning Health Minister Chris Hipkins announced a new suite of measures for aircrew to come into force next week.
Overseas-based air crew who are laying over in New Zealand will be required to stay in a Government managed isolation facility for as long as they are in the country, to ensure they avoid contact with New Zealanders
They will be required to wear PPE on flights, in airports and whenever travelling between the airport and their hotel. They will also isolate with other crew members while on overseas layovers.
New Zealand-based crew returning to New Zealand from high-risk locations overseas will be required to self-isolate for at least 48 hours after they return, and until they are assessed as being low risk of having Covid-19.
The new measures are part of a public health response order that will come into effect from 11.59pm this Sunday.
They were flagged over a week ago by director general of health Ashley Bloomfield.
It comes after it emerged yesterday that 12 people in managed isolation had tested positive for Covid-19 - 10 were on the same flight from India on September 26.
It has prompted Otago University epidemiologist Professor Michael Baker to push for a week in quarantine and a negative test before travellers can fly into New Zealand from Covid-risky countries.
The National Party has also been pushing for a pre-departure negative test, but Labour leader Jacinda Ardern remains lukewarm on a pre-departure test, saying it would "not necessarily" strengthen New Zealand's overall response.