The Ministry of Health has released more details around when people will be eligible to get their Covid vaccine booster shot now the wait has been reduced to four months post second dose.

It comes as there has been some confusion around when people can get them. On Wednesday, a 75-year-old woman told 1News she took herself and her disabled son to get a booster shot upon hearing Tuesday's announcement.

Both had received their second dose over four months ago.

However, the woman was denied by a healthcare worker, who told her she couldn't get one until January.

In a statement to 1News, the Ministry of Health says the rollout of booster shots for people who had already reached the four month mark will begin on January 5.

People who have received their second vaccine over six months ago are still able to get their booster shot now.

In the statement, the ministry said those who had reached the four month mark would be able to walk into any clinic without a booking for their booster shot from January 5, or can choose to wait until January 17 if they prefer to book.

Bookings will be made using the Book My Vaccine website.

"This period [between now and January 5] will allow for the necessary operational changes to be made by the ministry ahead of these key dates," the statement said.

"Some of these changes are complex and across multiple systems.

"This important work will ensure the continued safe and effective immunisation of New Zealanders."

The ministry said there needed to be changes to the systems that recorded all vaccinations, the booking system used to make appointments and safety monitoring systems, in order to recognise four-month boosters as a valid entry.

Training vaccinators and being able to meet demand were other reasons for the two-week delay.