"Ngamihi's achievement is massive," she said. "She's an icon around here and a role model who sets an example at the pool."
Mrs Stephenson took on the challenge to help with her breathing and initially thought because of her asthma she would do one length freestyle and backstroke on her second.
"On the second week I said to Kelly [Webber], the manager of the pool complex, I felt so good I would do freestyle all the way," she said. "At the start of the challenge I had my puffer at the end of the lane, but as my breathing improved, it was put away altogether.
"I'm 64, so if I can do this, then I'm sure a lot of young ones can, even if they've got asthma. It does help."
Although the challenge is over Mrs Stephenson doesn't intend to give up swimming. "I'm going to keep on and I'm even thinking about taking part in the Iron Maori in Hawke's Bay, too," she said.
After she completed her 100km on Friday, Mrs Stephenson was presented with a basket of goodies, including beauty products and a restaurant voucher.