Christmas plans for tens of thousands of people may become clearer tomorrow when Air New Zealand and union officials meet.

Workers have threatened to strike over pay and working conditions on December 21, the Friday before Christmas; one of the busiest flying days of the year.

The strike would affect up 42,000 international and domestic customers who are booked to travel that day.

And a travel expert said that the strike might flow-on to impact for other travellers leading up to the festive season.

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Air New Zealand, E Tū and the Aviation and Marine Engineers Association are due to enter negotiations tomorrow.

Nearly 1000 members of the unions, which represent aircraft maintenance engineers, aircraft logistics and related staff, plan to strike over cuts to members' conditions, including overtime rates.

The airline said the group had received pay increases annually for the past 12 years, and had rejected a recent offer of 2 per cent followed by a further 3 per cent increase after 12 months.

Its maintenance engineering and logistics staff earned an average income of $115,000 and about 170 earned more than $150,000, the company said.

This year Air NZ reported its second highest profit, $540 million before tax, and its chief executive Christopher Luxon earned about $4m.

The airline said if a deal could not be reached, it would get in touch with passengers when that was known.

Many people have been left scrambling with wedding and graduation plans as well as Christmas family reunions up in the air.

Raina Ng, who is to marry her fiance, Trent Young, a day later, said it was a nightmare scenario for them. She was trying to find a Plan B for guests from throughout New Zealand and Malaysia who were booked to fly into Wellington on the day of the threatened strike.

Several Canterbury University faculties also have graduation ceremonies that day.

Prime Minister Jacinda Ardern said she was "very concerned" that the strike would affect many travellers and urged parties to make every effort to prevent significant disruption.

Brent Thomas, commercial director for House of Travel, said the impact would continue well beyond December 21 and cause issues for people connecting with international flights.