Ms Callaghan, 38, had signed up to be part of a tour with quad-bike specialist Riverland Adventures.
It is understood members of the group are being re-interviewed by authorities following the death, and that the police serious crash unit is still carrying out investigations and liaising with Ms Callaghan's family overseas.
Ms Callaghan was on a tour with nine others. She was driving a bike, and was was found by others in the group, lying next to it.
The bike had tipped on its side and members of the group rushed to get her help.
A member of the group, who was one of the first to discover Ms Callaghan, said at the time that the scene looked "awful".
"I came around the corner and the bike was on its side and she was just lying there. So I stopped everyone else and went to her.
"She ended up quite close to her bike but I don't think it had rolled over her - it looked like she'd been flung from it."
The group member said she had spoken to Ms Callaghan earlier and learned that she had been on her way to Wellington to visit a friend, when she decided she wanted to take part in the tour.
Ms Callaghan had serious head injuries and was taken to Auckland City Hospital where she was put in an induced coma.
Yesterday Riverland Adventures owner, Percy Kukutai, refused to comment. At the time however, he told the Herald that following the accident, the company had been in touch with Ms Callaghan's family and friends in Australia.
"We regret that there has been an accident," he said in a statement.
It is understood the company has been offered support by Hamilton & Waikato Tourism.
The accident was the second involving a quad bike in the same week.
The day before Ms Callaghan's accident, Sam White, the son of a sharemilking family, was killed while riding on the family property.
He was found pinned beneath the bike by his father Scott.
A spokesman for the chief coroner said last night the accidents had brought on new debate surrounding quad bikes and whether or not more safety measures needed to be put in place to prevent more deaths.
According to data from Ministry of Innovation & Employment, formerly the Department of Labour, 850 people are injured riding quad bikes on farms every year and five die.
Quad bike deaths 2012
Oct: Chelsea Callaghan, of Melbourne, died at the weekend following an accident near Onewhero.
Oct: Shane William White, 10, died in the Wairarapa.
Jul: US man Jeffrey Robert Frum, 21, died at Hahei.
Mar: Luke James Randle, 20, died at Foxton Beach.
Jan: East Coast golfing identity Peter Rouse, 74, died on a Tokomaru Bay farm.
- additional reporting: Anna Leask.
