Unvaccinated Kiwis will now wait six weeks between their first and second Covid vaccine doses.

The decision comes alongside an acceleration of the Government's age-band eligibility system, which will allow anyone over 16 years old to book a vaccination appointment from September 1.

Director general of health Dr Ashley Bloomfield said the three-week gap between vaccines that almost 850,000 fully vaccinated Kiwis experienced would be doubled to ensure further protection against threatening variants.

"Moving to a longer gap allows us to give a first dose to a larger number of people faster, which means providing more people with partial protection sooner," he said.

"This is an important part of our being prepared for the possible outbreak of the more infectious Delta variant of the virus."

Director general of health Dr Ashley Bloomfield. (Photo / Mark Mitchell)

Bloomfield quoted studies which demonstrated an extended duration between jabs contributed to at least an "equally robust immune response" and could reduce the likelihood of rare side effects, myocarditis (heart wall inflammation) and pericarditis (swelling of membrane around the heart).

For those with an existing booking for their second jab, Bloomfield said they could either keep the appointment or re-book - emphasising the most important aspect was everyone receiving two doses.

He admitted the new timeline would not be suitable for everyone, citing immunosuppressed people and border workers who may require their second vaccine earlier.

Bloomfield confirmed the change wouldn't alter the Government's plan to complete the vaccine rollout by the end of the year, as evidenced by the move to accelerate the age-band eligibility framework.

When Prime Minister Jacinda Ardern revealed the national Book My Vaccine system, she anticipated any Kiwi would be able to book their jab in October, but this has now been moved to a month earlier.

Recently, it was confirmed that people 50 and over would be able to book an appointment from tomorrow.

Today, Bloomfield said people 40 and over could book from Wednesday (August 18), while people 30 and over could book a week later (August 25).

"District Health Boards continue to add capacity to their vaccination programmes and we are opening age bands sooner than expected because of the high level of interest and increasing system capacity," Bloomfield said.