Doctors at Wellington's Ora Toa centres will strike on Friday.
It's the second time they've walked off the job because of their employer's refusal to include dentists in a collective agreement.
Association of Salaried Medical Specialists spokesperson Lloyd Woods says it makes no sense.
"It beggars belief that any employer would choose to have their staff being forced to go on strike, rather than agree to something that is very simple," he told Newshub.
Mr Woods says the 21 doctors, employed by Te Rūnanga O Toa Rangatira, even agreed to a smaller salary increase if dentists were included.
"We said look, we'll take 0.7 percent less than what you're offering in terms of salary increase, given the dentists are covered. That was quite extraordinary, but that was refused."
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Mr Woods says it feels as though they're the only party at the table.
"We're trying really hard - this is just bloody-mindedness."
A letter to the Ora Toa PHO board in November urged them to recognise the dentists' skills and their part in the wider medical team. The board said it was an operational issue for management.
Te Rūnanga O Toa Rangatira said in December the agreement isn't tailored for dentists, so it would be inappropriate to include them.
December's strike was the first-ever by GPs. Junior doctors have taken industrial action in recent years over their difficult rosters.