The Council of Trade Unions says the Ports of Auckland is using cameras, security guards and other tactics to intimidate workers.
But the company says the union is simply attempting to deflect attention from the focus of mediation - which is to resolve a lengthy dispute.
Talks on Thursday finished with little resolution and will resume on Friday.
CTU president Helen Kelly describes the ports company as "out of control", saying it has banned workers from talking to the media.
""It has installed security cameras far and wide including in every cab, so drivers are sitting there all day with this camera staring them in the face.
"They've got security guards in the tea-rooms. They've told them not to wear their union badges and union stickers. It's just a pattern of intimidation of these workers."
Workers returned to the job last week, after weeks of strike action and short lock-out periods.
Maritime Union president Garry Parsloe told Checkpoint on Thursday that, when workers resumed their jobs, they were told they would be working under normal conditions.
Mr Parsloe says it has been anything but, and the company is going overboard with security at work, hiring guards and installing cameras to monitor employees.
Port admits leaking info, says union
The CTU says Ports of Auckland has acknowledged in a letter to leaking private information about one of its workers to a blogger.
It says the company sent a response to the Maritime Union about Cecil Walker, whose leave details were published on the internet in March this year.
The blog said Mr Walker took 21 weeks off on full pay in 2007 and 2008. However, the man says he needed the time off because his wife was terminally ill.
CTU president Helen Kelly says Ports of Auckland tries to justify in the letter why it released the information and admits it may have upset Mr Walker, but does not acknowledge any wrongdoing.
Ms Kelly says the letter has been passed on to the Privacy Commissioner.