"The company has an obligation to those staff already allocated work for the next 7 days and to ensure that staff are rostered at appropriate levels to service the limited number of ships scheduled to call in the next few weeks."
Health and safety reasons were also cited as a reason not to let union workers back into the ports.
Board chairman Richard Pearson this morning told Radio New Zealand he wanted the union to guarantee workers still at the port would not be subjected to intimidation from union members.
"We are obviously concerned about our staff that are already working at the port, they have been subjected to much intimidation and threats of physical violence," he said.
Maritime Union chairman Garry Parsloe said the decision to bar union wharfies from going back to work was "incompetence at the highest level".
He said the port was "crippled" and needed its full workforce back on site.
"We're not locked out and we're not on strike so we should be back there right now."
Mr Parsloe said the union was taking legal action against what it considers to be an illegal lockout.
He was this morning talking to union lawyers about what course to take.
Council of Trade Unions (CTU) president Helen Kelly said the union members were entitled to return to work and the lockout was unlawful.
"And for Mr Pearson to come up with another reason - continuing to accuse these very, very good people of being thugs and being violent - is absolutely outrageous. If he has got any evidence of this he should give it to the police,'' she told Radio New Zealand.
"There is no evidence of threats. Honestly, there's 300 workers - what, they're all running around threatening people?
"They haven't seen these workers inside the gate for three weeks, they've been on strike.''
Ports of Auckland issued its lockout notice a day after it scrapped plans to make all 292 union workers redundant and replace them with contracted stevedores.
Its decision to scrap the plans came after an informal meeting with the Employment Court.
