The port, which uses contractors for all its stevedoring, has asked its contractors for vaccination information.
But as ISO chief executive and director Paul Cameron says, that information isn't easy for contractors to get.
However, effectively everyone in the port industry knew that, from October 1, no one would be able to work on or around vessels without being vaccinated, he said.
This is due to a new vaccination order mandated by the Government for border workers which became law on July 15.
Meanwhile, getting official information about vaccinations was challenging.
He couldn't accurately say how many ISO employees were vaccinated "as the ... Government is still deciding the full extent of which workers need to be vaccinated.
"However we do know that our stevedores that board vessels or work around vessels must be vaccinated and there are over 800 of our people in these roles that could be affected.
"That said, ISO is still uncertain of just how many people the border order might extend to and ISO is seeking clarity on this from Maritime NZ and MoH."
Cameron said ISO had been working closely with MoH "for a long time now".
"The current challenge is getting accurate data from the BWTR but like every Covid challenge that the industry has faced, there is confidence this will be resolved.
"ISO has been providing evidence of vaccinations that don't show on the BWTR so this is an area that needs to be addressed."
When the Herald approached MoH on the information gap issue, it first pointed to the new compulsory vaccination order.
Group manager, Covid-19 testing and supply Darryl Carpenter said some workers now covered by the order were vaccinated voluntarily before the new requirement came into force.
The fact of their vaccination had been recorded on the register but the worker had to agree to their employer seeing that information.
Where consent wasn't given, the employer or business operator wasn't entitled to know the worker's vaccination status.
"Many of these workers will now be covered by the mandatory requirements ..."
Carpenter said the ministry had informed employers of the process to follow to update their workers' BWTR record so that vaccination records were visible to the business owner or operator.
"This is to protect the health information privacy of those no longer performing border work or who are no longer employed ..."
The groups of workers covered by the new order largely aligned with groups subject to mandatory testing under the 2020 Covid-19 public health response testing order, said the ministry.
Employers and business owner/operators had been advised to contact the BWTR team to confirm if a worker was covered by the new order.
"Once they do that the ministry will take the necessary steps to show the (employer) the vaccination status of those workers in the BWTR," said Carpenter.
ISO's Cameron said the company had been warning staff about mandatory vaccinations for border workers "for a very long time".
It was informing staff of the new requirements and criteria and offering support to book vaccinations, as well as providing information events.
"ISO is optimistic everyone will be vaccinated. We are talking about a group of employees that have been complying with Covid controls since March 2020. We are an essential industry and our people kept working through the lockdown.
"We've been subject to fortnightly nasal testing and we have been complying with rigid PPE and distancing regimes for the last 18 months so vaccinations are just another phase of the overall direction our staff and the business have to take."
However Cameron said a few employees might choose not to be vaccinated.
"... The one thing we know for sure is that they will not be able to work at the border after October 1 and therefore it is highly unlikely they would have a job with ISO after that date."
The Port of Tauranga has warned that border worker non-compliance with the vaccination order will likely worsen a labour shortage in the sector and lead to further cargo delays.
