She had been staying at a Lily Pond Road property at Haruru, east of Paihia, for a party when the Waitangi River burst its banks.
About 6am yesterday, the woman went to check on her car, which was partially submerged by rising flood waters, Ms Kennett said.
"And then she tried to get back to the house and I think that's when she got caught out in the current."
Three others were also swept away in trying to rescue the woman, and had to be rescued by emergency services.
The host of the party posted dramatic video of the flood on Facebook, telling friends as the drama unfolded: "No one has been saved. Still one guy on the shed roof. One on a car. Two gone down river. It's terrifying."
At 8.15am she posted: "Still two people missing. Apparently they have jet skis looking for them. Been in the water for two and a half hours now".
Later that morning, she said the searchers had: "Found everybody except for one girl".
Police said they would be "catching up" with the party organisers and others at the party.
When police and the fire service arrived at the scene they found another three people either stuck in trees or on top of car and shed roofs, Inspector Greg Dunn said.
"They began rescuing these people using helicopters, jet skis, IRBs and ropes and managed to rescue two people who had spent a long time in the water.
"The Westpac Rescue Helicopter and Skyworks Helicopter and Police Search and Rescue (SAR) staff attended the scene and also lifted one person from a shed roof."
The helicopters and SAR staff continued an air search of the Waitangi River in treacherous conditions in an effort to locate the missing woman, Mr Dunn said.
A nearby resident Henk Kuiper yesterday took footage of rescue helicopters searching for the woman near his home about 9am. "You don't normally see that sort of action around here," he said.
Another resident, who did not want to be named, said the area where the woman disappeared was a popular swimming hole.
Another local woman said the area was prone to flooding, and she was surprised the council had issued permits for properties to be built there.
Read more:
• Dramatic rescues from floods
• Rescuers abused by partygoers