"This was something that she wanted to do significantly - of all the things she could do in a 50-minute train ride radius, she chose this," Easther said.
The group set off in an order - Riverland Adventures owner, Percy Kukutai, was in front with a child on his bike, followed by Ms Callaghan, Easther with her son, then other parents with their children behind.
Towards the end of their trip, they took a one-way gravel narrow road.
"Then I came around the corner and the bike was on its side and she was just lying there. So I stopped stopped everyone else and went to her," Easther said.
Ms Callaghan was bleeding heavily from her head injury and the scene was "awful", Easther said.
There was no phone reception in the area and so while one of men stayed with Ms Callaghan, Mr Kukutai ran up the hill to try to get reception and Ms Easther took a bike to the main road to call an ambulance.
"I didn't even know where I was, I had to go up to people's letterboxes to get the address."
Easther said the accident was nobody's fault.
"She might have been enjoying the view and then over-corrected, you just don't know.
"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."
Mr Kukutai said he had been in touch with Ms Callaghan's friends and family in Australia.
"And our thoughts and prayers are with our dear lady."
He said he could not comment further as the police and Ministry of Business Innovation and Employment were investigating the accident.
