Mother-in-law Mary Lythe said the accident happened in Albany on Thursday.
The flying fox sloped up at the end, and Mardie was "flung off the end" as though she had gone over the handlebars of a bike.
Mary Lythe said the flying fox had been safely used by others, including children, and exactly why Mardie came off it was unclear.
"I don't know if they'll ever know, but it's unusual for somebody like her to let go. It's just a freaky accident, because she's a very athletic girl - she's used to all of these things."
Brother-in-law Nick Margison said doctors suspected Mardie had blacked out before she fell. But as she was still heavily sedated, they had not yet got the full story from her.
Mary Lythe said it was tragic for somebody so sporty to be unable to use her legs again. "I was looking at her lovely active body and I thought, 'Oh, how tragic. The legs won't move, those beautiful legs'."
Mary Lythe said her son was now focused on looking after the couple's four daughters - aged 7, 10, 13 and 16 - who were trying to stay positive.
"The kids have already looked up on the internet her events for the Paralympics."
Mary Lythe said her son had been strong and very practical. "He's quite resilient ..."
He planned to return to his outdoor landscaping job, but would have to reduce his hours to be there for the children.
The family live in a two-storey house in Milford, but Mary Lythe said they would need to find a single-storey house so Mardie could get around in a wheelchair.
They hoped to remain in the area because the children were all involved in sports there.
Mr Margison has set up a trust fund so his brother-in-law can take time off work to look after the children and find a rental home without stairs.
He said the children were devastated. "When she gets home it will be a massive change - they probably don't realise what a change it will be. But they're trying to stay positive."
Mr Margison said he was encouraged by support from Ben Lythe's old mates at the Warriors, the Mt Albert Lions and the Glenora Bears, who had contributed to the trust fund.
Former Warrior Awen Guttenbeil is working with the old boys' network to generate support, and Warriors CEO Wayne Scurrah said the club had donated $5000 to the fund.
League ambassador Sir Peter Leitch said Lythe was one of the most genuine and decent blokes around. "I'll be making a donation and I've also rung a few people to see if they can help too."
Donations
Trust fund account number 38-9007-0327920-01
Account name: Lythe.