In Dunedin, the service targets 274 donations a week, 164 of which are for plasma and 110 whole blood donations.
Now, as Omicron continues to take a hold in the community, those targets are looking hard to hit.
Next week, only 40% of whole blood donation appointments have been filled, while the week after that drops to only 26%.
Plasma donations are in slightly better shape.
The week starting March 14 has 80% of its appointments filled, dropping to 51% in the week starting March 21.
National marketing and communication manager Asuka Burge said the need for blood was constant, but the service was not immune to the effects of Omicron.
It was expecting increasing Covid-19 numbers to have a significant impact on the number of people eligible to give blood.
"Blood only lasts 35 days. This is why we are asking more healthy people to donate now to help ensure we have sufficient stock levels in preparation for the coming weeks and potential donor shortages."
New donors and the return of lapsed donors could be crucial to ensure a steady supply of blood in the coming weeks.
Potential donors needed to book an appointment to donate rather than just turn up to help enable social distancing and other Covid-19 precautions, and to help the service forecast its ongoing supply.
"Every appointment is vitally important, so we also need people who cannot attend an appointment to donate, to either reschedule or cancel their appointment so our team can try to re-book the appointment."
Dunedin clinical nurse leader Chrissy Bacher said all donors could expect a friendly and inviting environment when they walked through the doors of the donation centre.
The classic post-blood donation cup of tea and a biscuit or cheese and crackers remained popular with donors, she said.
