Just 47 per cent of those eligible for the second booster had taken up the chance - meaning about 780,000 people had not.
The first booster was at 73 per cent but because it had wider eligibility, that meant more than a million people had not had one.
The wasted doses were the original Pfizer vaccine but Te Whatu Ora was now offering a new bivalent jab for those getting a booster.
From Saturday, more people would have access to it.
Vaccinologist Helen Petousis-Harris said there were a lot of reasons people may have been slow to get boosted.
For example, some may have only recently had the virus, while others may have been holding out for the new vaccine.
It was important those at higher risk of harm from Covid - like older people - got as much protection as they could, she said.