Police don't believe she came back to the Auckland area alive but would not say what they found in a distinctive yellow ute that followed her from Papakura to Hamilton, or if Ms Portman was aware the ute followed her to the Waikato.
Her movements were traced by tracking her cellphone activity.
Detective Inspector Mark Gutry of Counties Manukau said the circumstances surrounding her death still needed to be established. But he confirmed police weren't acting on a tip when they visited the property, on Papakura-Clevedon Rd.
"When we went to the house we did not know we would be finding a body there ... We have arrested a man this week and as a result of the follow-up inquiries with that we came to this address."
He said police didn't know whether Ms Portman had any connection with the property but officers didn't think the people who lived on the property were aware the body was hidden nearby.
"We've spoken to the owners and they have co-operated with us and we do not believe they knew there was a body here."
Although the body hadn't been positively identified it was believed to be that of Ms Portman and Mr Gutry said it would be pleasing to return her to her family and offer them closure.
Asked whether any weapons were found near the body the detective said: "I'm not going into what was found near the body."
Mr Gutry said police were still seeking information from the public, especially movements of the yellow Holden Commodore ute with the licence plate LCKDUP and Ms Portman's black Mazda Astina.
"We believe those vehicles hold information about what happened to Rae and we're seeking information about their movements between June 20 and June 25."
The Mazda was found at a Hamilton wreckers' yard on June 25 but Mr Gutry would not say how it got there. "That's part of the investigation. We have identified the person who took it there but that's part of our inquiry."
The Hamilton man will reappear in court next month and has interim name suppression.
