Medsafe says it destroys about 70% of medicines which have been bought online and sent to New Zealand.
New Zealand Customs officials have seized 289 packages containing medicines in a week-long sting led by Interpol in dozens of countries.
Medicines for weight loss, diabetes and insomnia were the most prevalent, but most packages were unlabelled. Many came from India, the United States and China.
It is the sixth time that New Zealand agencies have been involved in the operation. Last year, 124 parcels were intercepted.
Medsafe said it co-operated with the crackdown on sub-standard, illegal or counterfeit products. Manager of compliance Derek Fitzgerald said it is illegal to buy prescription medicine without a doctor's prescription.
Mr Fitzgerald told Radio New Zealand's Checkpoint programme on Friday that the authority gives people being sent medicine a chance to prove that they are legally entitled to it.
"We're just asking them if they do have a reasonable excuse in the way of a prescription. If they don't, they can't have their products.
"Generally speaking, around about seventy percent of the products coming through are destroyed in the end because the intended purchaser doesn't get back to us or is not able to find a doctor who will give them a prescription."
Medsafe said people should consult their doctor on the potential side-effects of medicines they intend to buy online.
