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Delwen George (5), of Wellington sustained internal injuries and remains in ICU in Dunedin Hospital. His baby sister was also injured.
Their mother, Selma George, had a fractured femur and dislocated hip which required surgery and another male, understood to be the children's father, was also injured.
He and an unrelated male, treated at Lakes District Hospital, were both discharged yesterday.
The victims were mix of foreign nationals and New Zealand citizens, but all were residing in New Zealand, police said.
Delwen remained in a serious and stable condition. His mother and sister were stable.
Ngai Tahu Tourism chief executive Quinton Hall said there were some off-duty fire service members from Auckland coming off the boat who leapt in and helped out, along with staff and members of the public.
"The team responded quickly . . . they were able to ascertain pretty quickly [that] it was a serious incident and called emergency services straight away, that helped - they were there very quickly."
Mr Hall said it was not immediately clear whether Ngai Tahu Tourism - which operates the jet - was responsible for maintaining the willow, or the Department of Conservation, which owned the land.
An arborist had visited the site yesterday to inspect the tree as part of a report for authorities, including the police and Department of Conservation.
Mr Hall was not sure when the tree was last inspected, either visually or internally, or when a road beside the tree was constructed.
He said the group was ''just enjoying the time, watching the boats go by'' when ''tragically the tree fell'' about 1pm.
It's understood the family arrived in Queenstown for a holiday that morning.
Mr Hall said Ngai Tahu had three staff in Dunedin supporting the wider family, ''making sure they've got everything they need''.
''They've got a number of family [members] in New Zealand and they've arrived in Dunedin, so they're well supported at a number of levels.''
Staff in Queenstown had also been offered counselling, he said.
The company had suspended it operations and was yet to decide when it would resume them.