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Clarke Gayford has since apologised for "any issues or confusion" caused by the incident.
In a post on a private Facebook group, Tauranga pharmacist Michael Taylor said a group of vaccinated musicians had come into the store and requested RATs after having been potentially exposed to Covid-19.
Taylor said he explained the Ministry of Health rules required them to instead get a laboratory-based PCR test.
"As they didn't like this they got Clarke Gayford on the phone who proceeded to tell me that there had been a change in guidance and these people should be given RAT tests," the post said.
"When I explained that we had not received any direction from the MoH he was very unimpressed."
Gayford did not respond to RNZ's interview requests, but his manager issued a brief statement: "Mr Gayford was rung by a friend about rapid antigen testing and was put on speakerphone while the person was in a pharmacy.
"He apologises for any issues or confusion this may have caused the pharmacy staff."
A spokesperson for the prime minister declined to comment, instead referring RNZ to the statement issued on behalf of Gayford.
National Party Covid-19 spokesman Chris Bishop described the incident as "deeply inappropriate".
"He shouldn't be giving advice to pharmacists about how they administer public health rules. He's not a public health expert. He's the prime minister's fiancé.
"[His friends] clearly rang him for a reason, because they knew he would have some potential influence over the pharmacist, but he should have just declined to comment."
Bishop told RNZ the public deserved more answers from Ardern and Gayford.
"All we've got is two lines from Clarke Gayford and nothing from the prime minister. It would be good to have some greater transparency about what exactly has gone on here.
"We can't have a situation where mates of the prime minister's fiancé give him a call and try get to access to things they're not entitled to."
ACT leader David Seymour also published a tongue-in-cheek statement, commending Gayford for "advocating for rapid antigen tests".
"I'd never hold a politician responsible for their partner, but I hope the prime minister will lift the crazy restrictions on rapid antigen tests that her fiancé was trying to fight against."