Starship Hospital respiratory paediatrician and Association of Salaried Medical Specialists president Julian Vyas told RNZ's Morning Report members had not voted to take action on the basis of patients missing out.
"Patients would have waited a long time anyway. There's long delays, there's rationing of some services within the public health system.
"Patients have suffered a lot already, we're taking what we believe is limited action... yes it will affect patients, and we regret that deeply and we haven't done this on a whim."
According to Te Whatu Ora, the average base FTE salary for a senior medical officer was $238,500 (excluding additional salary payments for shift work, being on-call and superannuation).
The average Total FTE salary package for a senior medical officer is $318,000 (including additional salary payments and superannuation).
Te Whatu Ora has stood by its offer to increase their pay by between $15,000 and $26,000, as well as a lump sum payment of around $4000.
But the union said that would amount to an effective pay cut, given inflation.
Combined with staff shortages and unsustainable working conditions, the union said more and more staff are being driven to the private sector or overseas.
Vyas said it was hard to know how Te Whatu Ora arrived at a figure of $318,000.
"They've presented a figure to suggest that doctors earn large amounts of money... the issue for us though really is around the fact that our pay over the past couple of years has really fallen in real terms and we believe that on a point of principal that no worker in the public or private sector should be expected to take a real paycut."
Beyond this, part of the issue lies with being able to recruit or retain people in the public health sector, he said.
"For members, this CPI pay claim being rejected by Te Whatu Ora is really the last straw.
"We've spent years, we believe, propping up a health system that has been under resourced, understaffed for a long, long time and what's happened is our members have had to do more and more and more to plug those gaps."
Plugging the gaps had become the norm, he said.
Te Whatu Ora chief people officer Andrew Slater told Morning Report he stands by the pay offer and said it was fair.
"The strike action received yesterday is regrettable, this will not affect patient safety and we've got contingency plans to manage events like this."
It was not the end of negotiations and Te Whatu Ora was doing everything it could to avoid disruptions to patients, he said.
"I think the challenge for us within the organisation is to deal with a number of competing demands and so against that backdrop we feel that the offer that's currently on the table's fair, we're looking forward to continuing to talk to it."
Te Whatu Ora had recently published a workforce plan that clearly articulated there was a shortage, he said.
"Over the last 12 months Te Whatu Ora's managed to close that gap by over 200 senior medical officers and a further 400 as well."
Prime Minister Chris Hipkins, speaking to Morning Report, said the average salary figure of $318,000 provided context.
"We are proposing that they get somewhere between a $15,000 to a $26,000 a year pay increase.
"That is a sign that we value the work that they are doing, and we want to get back around the bargaining table and talk about any issues that they have."
National Party deputy leader Nicola Willis described the strike action as historic.
"These senior doctors haven't gone on strike like this. And I think what it tells you is despite all of the resource and effort that's gone into the Te Whatu Ora restructure, actually, those on the front line… are at peak frustration."
The senior doctors needed to be listened to and worked with to understand what was making them so frustrated, Willis said.
"But we do that in the context of a very tight fiscal situation with an economy in recession and major challenges across the economy.
"I have not seen the claim that the doctors have put forward yet, but there is something going very wrong when you've got senior doctors striking."
The first strike will take place on 5 September between 12pm and 2pm.
That will be followed by another two-hour strike on 13 September and a third four-hour strike on 21 September.
