Less than a quarter of those sent initial invitations to the country's first mass Covid-19 vaccination event booked a slot.

Health officials have had to scramble to find 12,000 more people so it can go ahead on Friday.

The three-day event starts Friday at the Vodafone Events Centre in Manukau with an aim to vaccinate at least 15,500 people.

It was supposed to be limited to Manukau Institute of Technology students and their families to stop it being overrun with vaccine hopefuls - but that did not turn out to be a problem.

About 12,500 students and teachers were sent invitations and told their families could book too.

Only about 3000 booked a place.

Auckland's district health boards say they then widened the invitation to 82,000 Māori and Pacific people, but they still did not get enough bookings.

They eventually sent a total of 140,000 invitations before they had the numbers they needed.

The head of the roll out for the DHBs, Alex Pimm, says his teams will work with the community to understand why the take-up is so slow.

"I think it is always disappointing when something that you've planned for doesn't get the response that you were hoping for. I think it is an opportunity to learn about how we run these events for New Zealand in the future," he says.

He does not think the initial low turnout is a concern for the country's wider roll-out, which is due to ramp up from tomorrow.

MIT was chosen because the officials hoped young Māori and Pacific students would encourage their families to get vaccinated, with South Auckland a higher risk area.

South Auckland general practitioner Dr Api Talemaitoga says the mass rollout is a great initiative, but Friday's event lacked Māori and Pasifika input.

The invitation was boring for event-loving south Aucklanders, he says.

"I think the organisers should have thought about making it a celebration or an event that is like a festival where people come along with their friends, can listen to a bit of music, or watch a bit of entertainment or dancing. Have a bit of food and then get vaccinated."

Talemaitoga says restricting vaccination access to a particular date and time is not practical for some families.

"It's over a weekend. They'll have to take kids to rugby. There's only one car in the family, so it'll be when that is available, so they can't actually make an appointment because they don't know when the car will be free, so we need to make it available for people to be able to walk in," he says.

More should be done to get GP practices online quicker as many people wouldn't be willing to get it anywhere else, he says.

MIT student council president Micah Sili says the relatively small take up could be because the announcement came out during semester breaks.

There is help available for anyone struggling to secure a spot and an online kōrero, featuring health professionals, community and student leadership, is being held in the morning to address people's concerns, she says.

"Part of the talanoa kōrero session is actually directing any of those questions that they have around maybe the booking, or maybe the transport issues so that we can sit down and address it and help them be comfortable when the week comes on Friday, Saturday and Sunday to get their vaccine."

MIT chief executive Gus Gilmore says a reminder email was sent to all staff and students yesterday and health board staff advised they had seen a surge in bookings yesterday afternoon.

"Mass vaccination is an innovative way of increasing South Auckland's resistance to Covid-19, as well as creating momentum in our community in support of vaccination. As a result, it is appropriate that the invite to be vaccinated be extended further than MIT whānau," he says.

"The conversations started around the event are highly valuable for community engagement."

Pimm says Auckland officials will look at how to do things better next time - including understanding why people did not book, whether the communication didn't work, whether the event was not convenient, and whether it was a mistake to initially invite a very targeted group.

A few hundred extra spaces had been added in case there were any MIT stragglers.

The clinic starts early on Friday and runs until Sunday, with each vaccinator aiming to inject one person a minute to reach the final target.

-RNZ/Rowan Quinn/Liu Chen.