Staff availability, passenger numbers and airport scheduling are the reasons why border staff can't check if all Australian travellers have tested negative for Covid-19 before entering the country.
And previous Government estimates that half of all incoming passengers are being checked appears to be the maximum, while accounts from people who have crossed the ditch suggest it is much lower.
Earlier this week, Covid-19 response minister Chris Hipkins publicly stated about 50 per cent of people were being checked at the border if they had evidence of their mandatory negative Covid test.
However, after multiple inquiries from the Herald, a spokesperson for Hipkins' office confirmed test checks varied across airports, were dependant on a range of factors and could be lower than 50 per cent.
"Customs has advised that it has been able to conduct short-term 'surges' of up to 50 per cent of arriving passengers depending on staff availability, and flight loadings and scheduling at each airport."
People travelling from Australia must test negative for the virus within 72 hours of their flight to New Zealand, as per current travel restrictions.
This did not apply to New South Wales or Victoria with quarantine-free travel was currently paused with those states due to rising case numbers.
More than 50 people had been caught without proof of a negative test and had been sent to a managed isolation/quarantine facility.
On Wednesday, the NZ Herald reported how NZ Customs checked pre-departure Covid tests through "random validation" - meaning not everyone was checked when the landed.
Act Party leader David Seymour felt this process presented substantial risk of importing the virus into New Zealand's community, saying it left the country a "sitting duck praying for luck".
Since Wednesday's article, many recent transtasman travellers had contacted the Herald to share their disbelief at how proof of their expensive Covid tests had not be asked for and, in some cases, dismissed.
Whangārei mother Ashley Rochelle returned from Australia to New Zealand on July 11 with her four children, her partner and her mother-in-law.
After paying more than $900 for Covid tests, Rochelle was not asked at any stage to provide evidence of the whānau's negative tests.
Upon landing in New Zealand, Rochelle asked a Customs official whether he wanted to see proof of her family's tests and he said no, allegedly saying only 7 per cent of people were being checked.
A Customs spokeswoman didn't respond to Rochelle's claim that only 7 per cent of people were being checked.
Auckland resident Nicola, who asked to keep her last name private, travelled to Melbourne with her husband and two children last week.
On arrival, they faced rigorous checking from officials if they had symptoms or had visited locations of interest.
However, on their return through Auckland Airport, Nicola said the Customs officer didn't request to see proof of their tests.
When asked why not, the officer allegedly said only a third of people were being checked.
"It's a waste of time if it's not being checked ... I'm not confident enough that there aren't holes in the system," Nicola said.
Michele Hawley flew to the Gold Coast with her husband to visit an ill relative at the end of June.
Before flying home to Auckland, Hawley offered their test and travel documentation to Australian customs staff, but none showed any interest.
That was despite an email Hawley received from the Ministry of Health which instructed them to show the documentation to airline staff before boarding.
In Auckland, a Customs officer failed to check the appropriate page of the Hawleys' test documents before granting them entry into the country.
"It just seems really slack," she said.
"Our issue was that if the New Zealand Government was so concerned that we had to stay over there for two weeks, then surely they would be checking everyone that was coming in."