The incident on a Perth-Auckland flight on May 21 followed "some tension" between the pair, after the flight's departure was delayed when the first officer was required to undertake a random drug and alcohol test, the airline said.
"This departure delay frustrated the captain who prides himself on operational efficiency," Air New Zealand's manager of operational integrity and safety, Errol Burtenshaw, told AFP in a statement Sunday.
During the overnight flight, the first officer left the flight deck for a break and then had coffee with a cabin crew member in the galley area.
When he tried to return to the cockpit the crew member spent up to two minutes attempting to call through to the captain to advise that the first officer was at the hijack secured cockpit door, but the calls were not answered.
"The captain did not respond or open the door because he was approaching a navigational waypoint and in his cockpit monitor saw a cabin crew member rather than the first officer ringing," Burtenshaw said.
"The first officer became concerned that the captain did not answer the calls and used an alternative entry method to gain access."
Burtenshaw said the airline had conducted an investigation into the incident and a report had been sent to aviation authorities.
Following the incident, the captain was stood down for two weeks and the first officer for one week.
Both have undergone counselling and additional training, Burtenshaw said.
The cabin crew, who "became quite anxious" about the inability to contact the captain, were also offered counselling.