Ports of Auckland workers manning a picket line outside their workplace on the first day of three weeks of strikes are adamant they won't be backing down.

The Maritime Union and the company are at loggerheads over the port's insistence on more flexible working hours, a proposal the union says is unfair and provides too much uncertainty for staff.

Harry Mayn, 61, who has worked at the port for 35 years, told NZ Newswire from the picket line on Friday that the port's chief executive Tony Gibson has a hidden agenda.

"It's obvious that the port wants to crush the union and get the union out of here and contract out our jobs."

The crane fitter, one of about 80 on the picket line, says the strikes are unsettling for staff, especially those with young families.

He says past disputes had been lengthier but the current situation was by far the worst when it came to the port's unwillingness to negotiate.

"And it's also the most destructive in the way of them threatening us with redundancies and contracting out."

Father-of-three Jermaine Witehira, 31, a port employee for 13 years, says staff just want to "keep doing what we're doing".

"It can be hard, but we've got a lot of people out there supporting us," the stevedore told NZ Newswire while standing on the picket line holding a "my family is worth it" sign.

"We've known this has been coming for the last couple of weeks so we've all saved a little bit of money."

Mr Witehira says the staff are already flexible enough and the company is not genuine in negotiating.

"I think they're trying to get the union out. It's not about money, we're on less money ... and the port is really productive."

Ports boss Tony Gibson says the industrial action is hindering the chance of reaching a deal.

"These repeated strikes only serve to make it difficult to set dates for further mediation," he said.