Five people who tested positive for Covid-19 in New Zealand's isolation facilities had negative results before leaving India.
The five new cases detected in isolation yesterday stayed at an isolation hotel in New Delhi for three days, before their flight to Christchurch - and were all tested before being allowed on the plane.
The National Party wants to make it mandatory for people to be tested before they leave another country to fly to New Zealand.
But University of Auckland medical professor Des Gorman said the latest cases demonstrated why such a testing regime would not be a silver bullet for the highly infectious virus.
Rohit Sharma was on the flight from India via Fiji, which landed in Christchurch on 27 August.
Along with fellow passengers, he had been in quarantine in New Delhi, where everyone was required to be tested for Covid-19.
"Those who [tested] positive weren't able to catch that flight, only those who had a negative Covid test boarded," he told RNZ's First Up.
It came as a big surprise to Sharma that five of those who were on the plane with him returned positive Covid tests once in New Zealand.
"They went through the New Delhi airport. They may have been in contact with someone there, or maybe they have used washrooms," he said.
"Everyone had a mask on, but we still can't say where they caught this virus."
RNZ has seen proof of Sharma's Covid-19 test on 24 August, which shows he had no detectable signs of the virus.
He has also tested negative since arriving in New Zealand.
Professor Gorman said Sharma's story highlighted the need for layers of defences against the coronavirus.
"One of the explanations of course is that they were infected in transit. The other one is that they had a false negative test when they left India. So I think from a public health point of view you would not see pre-flight testing as a single line of defence - it's one of the multiple lines of defence," Gorman said.
"But the public health argument is if you are infected with this disease you should be in quarantine, not going through boarding lounges, sitting on planes with other people, regardless of attempts at social distancing and wearing masks."
He supports mandatory testing prior to travel as one part of New Zealand's arsenal to protect people from Covid.
"If we are not to completely trash our economy, if we're not to trash our livelihoods and our wellbeing, then we need to have as secure a border as we can to minimise the likelihood of having to shut society down.
"The only way to do that is to have multiple lines of defence. Pre-testing is just one of those lines of defence."
As of yesterday, there were 132 active Covid-19 cases in New Zealand and 33 of these were imported cases in quarantine facilities, while 99 were community cases.