The government allowed the Avatar film crew to come to New Zealand when no criteria for it existed in immigration rules.
New rules were published on Tuesday and were backdated to 2 June, but the crew and other workers were allowed exemptions to the border closure last month.
Immigration lawyer James McLeod said the original criteria from March did not allow for exceptions on the grounds of significant economic value.
"I don't see how the decision to allow the Avatar crew into New Zealand could have been made pursuant to the immigration instructions in force at the time, because the definition of essential worker was much narrower then," he said.
"It involved delivering a response to the Covid-19 crisis, and/or maintaining critical infrastructure, neither of which I would consider a Hollywood film would fall within.
"So we have a government that has published a list of exceptions to the travel ban and then a government minister, other than the minister of immigration has made an exception to their published list, applying unpublished criteria, for example the significant economic value exemption doesn't actually appear anywhere in the immigration instructions."
The courts had made it clear that immigration instructions were not gospel, he said.
"That said, they give a definition of who is eligible for a visa entry permission and who is not. And you have a whole group of people here who are said to meet the definition when they don't, and begs the question as to who has made that decision by what power."
The Ministry of Business, Innovation and Employment (MBIE) said in a statement that due to an oversight by itself, immigration instructions were not updated in line with Cabinet's April decision.
"On 21 April, ministers with power to act agreed to further delegate decision making on exceptions to border restrictions for 'other essential workers' to the minister for economic development and the relevant portfolio minister," said its general manager of Labour and Immigration Policy.
"Immigration instructions now reflect that the minister of economic development in consultation with the relevant portfolio minister has the authority to determine who is considered an 'other essential worker'."
The immigration minister announced a softening of the border restrictions this week, with partners and dependants able to enter the country without having to be accompanied by their New Zealand-resident relatives.
It also allowed maritime vessels in under specific circumstances and granted permission to two America's Cup crews, including their families.
Immigration Minister Iain Lees-Galloway said: "We are working on a longer-term border strategy and we are exploring how we can create an isolation system that could support further opening of New Zealand's borders, for example for current holders of temporary work visas and international students, while continuing to effectively manage health risks from overseas arrivals".
In a Cabinet paper, ministers set out the possible criteria for essential workers: Those needed for less than six months must have unique experience and technical or specialist skills that are not obtainable in New Zealand, or be needed for a major infrastructure project, event of national or regional importance, or government-approved programme.
For a longer-term role, the worker must meet one of those short-term same criteria as well as earn twice the median salary, with certain exceptions.