Chemists will only be able to dispense one month's worth of medicine at a time, or three months of oral contraceptives, thanks to people hoarding prescriptions amidst the coronavirus pandemic.
The change will come into force at midnight.
Pharmac's chief executive Sarah Fitt says although there were no significant medicine shortages in New Zealand people were stockpiling prescriptions, making it difficult to restock quickly.
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"Unlike supermarkets, also under pressure from stockpiling but able to restock from New Zealand manufacturers, medicines are mostly sourced from overseas suppliers and the ability to restock quickly is very limited.
"We are making this change to ensure that remaining medicines stocks in wholesalers, distributors and community pharmacies are appropriately managed."
New Zealand is now in total lockdown to stop the spread of Covid-19, with 78 new confirmed or probable cases announced today. The country now has a total of 283 cases.
Lockdown means people must stay in isolation in their homes, going out only to access essential services such as supermarkets and pharmacies or to exercise.
Pharmac's decision was not made lightly, as it meant people in isolation could need to go to pharmacies more often, Fitt said.
"However, despite messages to prescribers, pharmacies and the general public, stockpiling behaviour has not slowed down. This is putting the fair and equitable distribution of medicines at risk," Fitt said.
Exceptions can be made for people with mobility issues, people who live rurally, the immunocompromised and the elderly.
Prescriptions will still be written in the same way and cost the same, and people won't need to visit their GP or prescriber more frequently.
Pharmac says the impact of Covid-19 is likely to affect the manufacture and supply of medicines for the rest of 2020 and possibly beyond, and it will be watching the global situation closely.
Director-General of Health Dr Ashley Bloomfield told media this afternoon that all people should be able to access pharmaceuticals they need.
He added that there were no significant shortages at the moment, but the measure was a precaution to ensure supplies of essential medicines.
"These are unprecedented times," he said.
Meanwhile, some contraceptive pills have also been hit by shortages but they're not because of the coronavirus.
Pharmaceutical giant Pfizer has informed Pharmac of a short-term supply issue affecting two common oral contraceptives - the combined pill Norimin, and progestogen-only pill Noriday.
Lisa Williams, Pharmac's director of operations, said the stock issue was not related to Covid-19.
"The supplier is managing the situation and has confirmed it has a delivery due mid-April which will take stock levels back to normal."
Williams said when Pharmac was advised there may be a shortage of a medicine measures were taken to ensure stock was available to New Zealanders.
Limiting dispensing to three months rather than six would help bridge the gap until fresh stock arrived in April.