On Friday, a needle previously used to vaccinate one person was used again.
Once staff at the Christchurch Hospital clinic realised the mistake, they immediately advised the second person, Canterbury District Health Board Senior Responsible Officer for the COVID-19 response Dr Helen Skinner said.
She said the DHB was in contact with all three people involved and blood samples had been taken from those who received the injection.
The DHB has apologised to both people.
A comprehensive investigation into how the incident occurred was under way, she said.
"We are doing everything we can to support all three people involved in this incident. We will continue to communicate with the person injected with a pre-used needle, supporting them and sharing any new information we receive.
"We would like to thank the staff involved for their quick actions, open communication with everyone affected and their co-operation in the current investigation into this issue," she said.
"If there are improvements we can make to our systems to reduce the chance of this happening anywhere else, we will certainly be implementing them."
