In the past few weeks Covid hospitalisations have crept back up towards levels seen at the start of the year.
Now summer social gatherings could further increase infections and put vulnerable populations at risk, Te Whatu Ora Director of Public Health Dr Nicholas Jones said.
"Wastewater is still showing elevated levels of the virus... It's likely to continue, probably, through to at least Christmas," he told Morning Report.
The booster is being recommended for anyone aged 65 or over, Māori and Pacific people aged 50 and older, the immunocompromised, and pregnant women.
He said reminders had already been sent to those over 75, and were now being extended.
"These groups are recommended to have a summer 2023-24 booster irrespective of the number of previous doses they've had, so long as they have completed the initial two doses and it's been more than six months since their last booster or Covid-19 infection."
There were 27 further deaths attributed to the virus and 7880 new cases reported on Monday.
Meanwhile, new versions of two Covid-19 vaccines are up for approval by Medsafe.
Last week, the organisation said it had accepted applications for the latest updates of the Pfizer and Novovax vaccines which cover the XBB 1.5 strain of Omicron, which dominated during winter.
