Around 1600 junior doctors may be available to work during a proposed strike, union numbers show.
Junior doctors who are union members will strike for two days on 18 October, after negotiations over their work hours broke down.
The New Zealand Resident Doctors Association wants the maximum number of days worked in a row cut from 12 to 10, and the number of night shifts reduced from seven to four.
According to a table released by district health boards (DHBs), total junior doctor membership across all 20 DHBs is 2520.
The union's total headcount is 4132, which includes locums or temporary doctors, and casuals.
DHBs say that means more than 1000 junior doctors are not part of the union, and will not be on strike.
Union membership is highest in Northland and Tairawhiti at 80 percent, compared to 51 percent at Capital and Coast in Wellington.
Membership at Waitemata sits at 65 percent, 55 percent at Auckland, 63 percent at Counties-Manukau, 62 percent at Waikato, 64 percent in Christchurch and 68 percent at the Southern DHB.
The proposed strike is expected to impact elective surgeries and clinics.