According to its website, its pharmaceutical facilities is a licensed manufacturing facility for both Medsafe and its Australian equivalent, the Therapeutic Goods Administration.
Its over-the-counter products include the Health Basics, Only Good and Home Essentials.
A spokesman for the company declined to comment beyond a statement to the ASX.
"By moving to outsourced contract manufacturing we will generate lower cost of goods and have greater continuity in product supply, both of which have been impeded by Covid-related impacts," chief executive Richard Vincent said in a statement.
In an email to Food & Grocery Council chief executive Katherine Rich, the API said the impact of Covid-19 and the "continual unfavourable ranging decisions by grocery retailers has made this an easy decision".
The decision was "just another example of how the current market duopoly has a very real impact on the fabric of our economy," Rich said.
"Margin expectations are so high, often it makes no sense to manufacture in New Zealand."
API also told suppliers it had been hurt by changing consumer habits and very low demand for cold and flu medicine.
Earlier this week, API rejected an A$1.38 a share offer to buy the company from retail conglomerate Wesfarmers, the owners of Bunnings and KMart.