"Some people might find it funny, but the larger picture has to be considered," he said.
"Even if the employees were off duty, they were in uniform, and one might think if this is how they act when they're off duty, how will they act when they're on duty.
"Working for an airline is a serious matter; there are serious consequences if things go wrong."
Gelb believed it would be in the employees' and Air New Zealand's best interests if the employees were to resign.
"The employees need to think about what would be best for them long term. If they have a termination on their record, that will follow them to their future job applications.
"From Air New Zealand's perspective, they'll want to get it all wrapped up and done with."
Gelb said employee antics similar to the airline staff were surprisingly common.
"I'm constantly amazed at the antics people get up to in the workplace. People exposing themselves, taking images of colleagues while they're in compromising positions just for a joke and so on.
"The Air New Zealand matter might seem like an isolated incident because it's been in the media, but in my experience, such behaviour is rather common."
Air New Zealand chief executive Christopher Luxon said the bad behaviour by crew posted to social media let down the entire company.
New Zealand Air Line Pilots' Association general manager Dawn Handforth said the association was aware of the matter but it could not comment on behalf of the employees.
"[The association] is prepared to give support to the airline and any pilot member, as required, once all the facts have been established."
A spokesman for E tū, a union which represents flight attendants, said it could not comment on individual employment matters while investigations were ongoing.