Mr Williams said he had "no fear" before his 30-second tandem freefall from 10,000 feet. "I'd be more scared of falling at home than falling out of a plane."
"Moira would have said 'You're mad," Mr Williams laughed.
"My opinion: 90 is a number. Your age is how you can live."
Mr Williams said he "might take on" a hang glider flight next.
After a trip through the Canadian Rockies by car, train and boat earlier this year -- which he ended up completing my himself -- Mr Williams told APNZ: "I've just about done my dash."
"He's a legend, he's amazing," daughter Mrs Low, 60, said.
The Rockies trip "took me 13 insurance companies to get him any insurance," Mrs Low said, "but it gave him a whole new lease of life."
Mrs Low said last Sunday's skydive "was a last minute decision."
"He was just expecting to watch me from the ground. There was a wait, he saw a few people coming down and he said 'I can do this' and I said 'I don't think so' and he said 'Why not?'
"When they were giving him instructions he said "I'm too old to remember all that!"
"He's an amazing man -- he still works for volunteering organisations in town."
Mrs Low agreed her mother Moira "would have been absolutely so against" the jump.
"She would've thought it was crazy. She would not have been happy at all."
Jo Sutcliffe of Skydive Bay of Islands said while people in their 70s are increasingly undertaking skydiving, Mr Williams was the oldest jumper the company has ever had.
"We've got an 80-year-old jumping today. She'd been inspired by our 90-year-old.
"People think it's going to be like a rollercoaster but it's actually just stable. You're just falling through the air. Ian knew it wasn't going to be a rollercoaster."