Wilson said the man she had known since the age of 14 was "vaccine-hesitant" but decided to get the jab after a celebratory breakfast following the purchase of their dream home.
"It was the worst decision of our lives," Wilson said.
Later that night at a family dinner, Nairn mentioned his chest felt "weird" but seemed unconcerned.
Over the next 12 days he experienced further chest flutters, as well as headaches and a sore elbow, but he put it down to the stress of the impending marriage and moving house.
"I went quickly from planning a wedding to planning a funeral," Wilson said.
Coroner Sue Johnson told the packed courtroom that she was not there to determine fault and that evaluating the benefits of vaccination or otherwise was "completely out of my jurisdiction".
It was accepted Nairn's death was caused by myocarditis and that was likely due to receiving the Pfizer vaccine.
The role of the inquiry was not to find blame or debate the merits of vaccination but to establish the facts and consider recommendations that could avoid further such deaths, she said.
The manager of the pharmacy where Nairn was vaccinated said medical professionals involved in the vaccine rollout had been informed about myocarditis, but the message was lost in the sheer volume of information they received.
She had been so busy she spent every evening reading the "millions" of emails from health authorities once she had finished eating dinner.
Sometimes "a link within a link within a link" contained important information, she said.
The pharmacist criticised the Immunisation Advisory Centre and the Ministry of Health, saying that information about myocarditis should not have been put in such small bullet points.
As medical professionals, they had no desire to miss information that could help the public, she said.
The vaccinating pharmacist, whose name was also suppressed, said patients at the time were not warned about myocarditis because of its rarity.
She was aware of it as a side effect, although she did not know it could be fatal. She had not been instructed by her employer to inform clients about it as a specific risk, she said.
On cross-examination by Wilson's counsel, the pharmacist broke down in tears.
"As soon as we were notified [of his death], all practices changed ... [It is] unfortunate Mr Nairn had to pass away for that to happen," she said.
Wilson told the court Nairn woke up in discomfort early on the morning of November 17 and said he would see a doctor the next day.
When he rose again a couple of hours later, he agreed to go to hospital. Wilson said she heard a heaving sound, which she thought was Nairn vomiting but found he had collapsed in the bathroom.
She could see his unresponsive body through the crack in the door but was unable to get in.
Emergency services performed CPR in the lounge for 45 minutes without success.
Her agony over her partner's death was compounded by a barrage of online abuse, because of the controversial nature of Nairn's death, and her family had been "blown apart" by the divisive vaccine issue.
The inquiry is expected to conclude next week.
