Second booster shots for people deemed at most risk of getting seriously sick from Covid-19 could be rolled out soon.

The Government has introduced a bill to amend the Medicines Act. The amendment enables voluntary booster doses to be administered without a prescription.

Health Minister Andrew Little said the changes would make it easier for people without access to a GP to get second booster doses.

Little said the changes would apply to about 850,000 people at high risk of getting very sick from Covid-19.

The bill will go to the health select committee and be reported back on June 20. The House of Representatives will consider the bill again that week.

Little said guidance on a second booster followed the health ministry's vaccine technical advisory group feedback on waning immunity after a booster dose.

"The majority of people in these groups will become eligible from July, following advice that six months is an appropriate gap between doses," the Health Minister added.

Little told the House further doses were required for at-risk groups, including the elderly.

"As we head into winter, the usual seasonal respiratory illnesses, alongside ongoing widespread Covid-19 infection, presents significant risk of severe outcomes for the most vulnerable."

He said winter would bring increased pressure on a health system already stretched due to Covid-19.

National's Covid-19 response spokesman Chris Bishop said his party supported the bill going to a select committee.

But Bishop said the committee should investigate the scientific rationale for the fourth dose.

National's health spokesman Dr Shane Reti said some overseas evidence showed a second booster could be beneficial to people most vulnerable to Covid-19.

He said that especially included the immunocompromised and older people.

Green Party list MP Teanau Tuiono said most people would become eligible in July for the second booster.

But because the booster programme started in late November, others would be eligible from mid-June.

"Therefore, there has been a push for this to go through urgency, and I totally get that because of the quickly evolving nature of the Covid pandemic and the need to actually do that," he said.

"But also ... it's good to actually get that detail so that the public has that information and it kind of alleviates some of the anxiety out there," Tuiono added.

Act Party leader David Seymour told the House 834,000 people were said to be vulnerable for a variety of reasons.

Seymour said pharmaceutical company Pfizer should have submitted relevant data for the booster's approval.

"But it seems, for some reason that hasn't been given to us by the Government advancing this bill, that Pfizer are not approaching countries with a rationale for a fourth dose to be allowed under their regulatory systems."

The Bill was read a first time and referred to the Health Committee.

Associate Covid-19 Response Minister Ayesha Verrall said a booster was especially important to people most at risk as winter approached.

"There is sufficient Pfizer vaccine in stock with current supply sitting at around two million doses," Verrall added.

Proposed eligible groups were people aged 65 or older, Māori and Pacific people aged 50 or older, aged care and disability care facility residents and severely immunocompromised people who'd received a three-dose primary course and first booster.

"Getting a booster reduces the likelihood of a more serious infection and helps keep people out of hospital," Verrall said.