The testing of border workers has once again come under fire after revelations Case B, a security guard at Auckland's Grand Millennium managed isolation facility, went almost five months without a COVID-19 test.

Prime Minister Jacinda Ardern is blaming the security guard for lying - despite an investigation still being underway. And the Government has no idea how big the problem could be. 

The 'L' word is not used lightly at Parliament. But the Prime Minister didn't hold back on Wednesday from hurling Case B under the bus. 

"It is obviously quite difficult when an individual, as we've been advised, has lied about being tested," Ardern said in the House, prompting gasps from Opposition MPs. 

Case B, a 24-year-old, was meant to undergo mandatory fortnightly testing. But according to Ministry of Business, Innovation and Employment (MBIE) chief executive Carolyn Tremain, who spoke to the Health Select Committee, the security staffer was last tested for COVID-19 in November. 

As it has been nearly five months, Case B has missed at least seven tests. 

The Government claims Case B told First Security, his employer, he was being tested - even though he wasn't. 

The Government doesn't know how many workers have missed mandatory tests. 

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"We don't know the exact number," said MBIE deputy chief executive Megan Main. 

That's because they've been running an ad hoc paper-based system where employers keep their own records. The border order requiring workers to be tested regularly was implemented in September, but with few checks it was being enforced.

"Carolyn wrote to all of the employers who work in MIQ in March to reiterate their responsibilities," said Main. 

But it was just a letter - and seven months after the testing rules were put in place. 

"We don't have a systematic way until next week of being able to write a query into the system to give us information," said Tremain. 

National leader Judith Collins has little faith in the system. 

"I don't think we can trust anything we're told when it comes to assurances on testing," she said. 

Ardern said it's a "very particular" circumstance. 

Despite absolving itself of any blame, the Government has come up with a fix. From Monday, all 300 border worker employers will have to use a testing register.

"As the register becomes mandatory, we'll get a better indication of whether there are any other people that have been doing that," said COVID-19 Response Minister Chris Hipkins.

"The overwhelming evidence that we've seen so far is that people are complying and they are being tested regularly."

Branding Case B a liar echoes the treatment of a COVID-positive KFC worker in Auckland, who was pilloried online after the Prime Minister blamed her for failing to isolate. The worker argued she'd never been told to. 

Officials are still investigating why the security guard wasn't tested. But, a bit like the KFC worker, that hasn't stopped the Prime Minister publically apportioning blame.