It comes after earlier strikes over deadlocked pay negotiations between their union and Te Whatu Ora.
The health agency earlier said it had put a fair deal on the table, but the Association of Salaried Medical Specialists argued the increase on offer of $15,000 to $26,000 failed to keep up with inflation.
In a statement on Friday, ASMS said its members voted "overwhelmingly" for two further strikes in October.
The 12-hour strike will take place on 2 October while the 24-hour strike will be on 24 October.
ASMS executive director Sarah Dalton said: "It's time for Te Whatu Ora to walk its talk about valuing senior doctors and dentists.
"Doctors want quality time with patients and whānau, not more overtime. Te Whatu Ora's refusal to provide an inflation adjusted salary is rewarding specialists going the extra mile with a pay cut."
Dalton said doctors were experiencing high levels of burnout, overwork and moral injury ad they tried to cover "massive workforce gaps".
She said Te Whatu Ora needed to show that the work done in public hospitals by doctors and dentists was valued.
"There are many issues confronting our health system, but salary is one problem Te Whatu Ora could solve today if they chose."
Dalton said the decision to continue strike action had not been taken lightly by its members.
"However, they see services unable to operate as they should every day - now they are standing up to keep doctors in our struggling hospitals."
Urgent care would still be provided to patients during strikes.
"We want to thank the public who have supported our doctors and dentists on the picket lines and who support our fight for decent investment in our health workforce."
The two two-hour strikes held this month were the first time specialists had taken nationwide industrial action.
The next formal mediated talks are planned for 19 September.
Strikes will delay treatment for more than 2000 - Te Whatu Ora
Te Whatu Ora said more than 2000 patients would have their treatment delayed as a result of these further strikes.
Chief executive Margie Apa said she was "extremely disappointed" that the senior doctors' union was escalating industrial action without letting its members know the details of its current pay offer.
"We have looked at every possibility to further improve the value of the offer, but to do this would involve finding it from elsewhere and we would need to do that work - something that is very difficult.
"We greatly value the work of our senior doctors and dentists and believe the offer presented to the union should be put in front of members - it includes salary increases over the next year of between 7 and 12.9 percent, amounting to between $15,000 and $26,000.
"We will be communicating this to the union at the next mediated talks on Tuesday."
Te Whatu Ora was "very concerned" about the impact on patients, she said.
"Our planning teams have advised that the 12-hour strike will disrupt planned care for 500 patients, with 200 of them already waiting a long time.
"The 24-hour strike will have a much greater impact, with up to 1700 patients' care deferred, 350 of whom have been waiting for quite some time. That's on top of 500 whose care has already been disrupted by the two strikes to date.
"That's a real shame and will no doubt cause frustration and anxiety for those affected and their whānau."
While hospitals were trying to rebook affected patients as soon as possible, some were likely to be waiting weeks or even months.
"Our goal to have no one waiting longer than 12 months for surgery by the end of December, except for orthopaedic, may be difficult to achieve now."
Meanwhile, contingency planning continued for a four-hour strike that had already been scheduled for 21 September from 10am-2pm.
