Whanganui District Health Board has apologised for a privacy breach that accidentally disclosed the email addresses of about 200 people.
The health board has been inviting people in Group 3 for their Covid-19 vaccine via text, letter and email, but last week an email was sent without the use of the blind copy function.
Recipients of the email were able to see other people's email addresses.
Human error was to blame, the DHB's Covid lead, Louise Allsop said.
"There was no other identifying information in the email except email addresses," she said.
"We have reviewed our processes to minimise this happening in future.
"Whanganui District Health Board is sorry that this occurred."
Group 3 of the WDHB's vaccination rollout includes people over the age of 65, Māori and Pasifika over the age of 50, and people with an underlying medical condition and disability.
A letter of apology was sent on Friday, stating that those affected had the right to make a formal complaint to either the health board itself or to the Privacy Commissioner.
Meanwhile a crucial vaccine shipment is expected to arrive in the country today.
Last week, Covid-19 Response Minister Chris Hipkins warned that vaccine stocks will be at "almost zero" around today, before a shipment of 150,000 Pfizer doses arrives tomorrow morning.
In total, the country is due to receive one million doses this month. However, getting tomorrow's vaccines from farm to table - or runway to arm - for New Zealand's 20 District Health Boards in quick fashion could prove difficult.
Nevertheless, those in charge of Auckland's vaccine rollout - the Northern Region Health Co-ordination Centre - are not anticipating any delays as no changes have been made to bookings at any community vaccination centres.
• If you are in Group 3 and haven't had your Covid 19 vaccine, call 0800 888 479 or email firstname.lastname@example.org to make an appointment.