A travel booking expert anticipates massive issues for Air New Zealand should a strike planned for December 21 go ahead.

Brent Thomas, the commercial director for House of Travel, told Newstalk ZB this morning that the impact is likely to extend well beyond the day of strike action.

"If travel is disrupted on the 21st December, that's going to have a flow-on impact into the 22nd and beyond," Thomas said.

"The 22nd through to the 24th will be close to fully booked already. The availability of seats on those flights is going to be fairly limited."

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Thomas said the only option would be for Air New Zealand to up-gauge existing flights to add capacity or to add new flights to carry the customers.

"It would cause a significant number of issues, not only for those flying domestically but also those looking to connect with international flights."

Air New Zealand said today that December 21 is expected to be the busiest day of the year, with 42,000 customers booked to travel on that day. The days around this are also expected to be highly congested.

The strike action is set to involve almost 1000 staff members, including aircraft maintenance engineers and aircraft logistics staff.

These staff members play an important role in ensuring Air New Zealand planes meet the required safety standards to reach their destinations.

Without them on the job, Air New Zealand will not be able to conduct the requisite safety checks before take-off.

The aviation unions and Air New Zealand will consult in the coming days, and Thomas hopes the parties find a resolution.

Air New Zealand has not yet rebooked customers, saying that it hopes to resolve the issue before taking the last-ditch step of shifting customers to different days and seats.

"I hope some sensibility comes through in the next day or two," Thomas said.