She drove about 110km north to Otara in South Auckland and was there by 2am, say police who tracked her cellphone. That means she would have needed to drive at 100km/h to reach Te Puna by 3.45am, to where her mobile was also tracked, before she arrived in the Mount.
Later, police forbade the woman from driving for 12 hours while her medical condition and suitability to hold a driver's licence were assessed.
Waikato police communications manager Andrew McAlley said it was too early to say if she would be charged.
It was also unclear whether police could apply for an injunction to keep her off the roads longer term - despite revelations that yesterday's trip was the second time in less than a year she had driven while asleep.
Senior Sergeant Dave Litton said police were first alerted when one of the woman's friends made a 111 call shortly after midnight telling them the woman had taken sleeping pills and driven from her home. "The woman's friend reported the driver had a sleeping disorder and had previously driven off while asleep 10 months ago, ending up in Tauranga."
Mr Litton said police established the woman's cellphone was on - and she was texting as she drove - but people receiving her messages believed she was half asleep. "When woken, she had absolutely no recollection of the events overnight and we have sought an urgent order forbidding her to drive and to seek medical advice on her suitability to remain holding her driver's licence.
"While her being found safe and well is a relief for everyone involved, the potential for tragedy was huge and we're urging people suffering medical conditions to be open and honest with their doctors and seek advice if the medication they are prescribed affects their ability to drive."
Sleep experts are stunned by the case, though Dr Tony Fernando, of the University of Auckland, said he was aware of several sleep-driving incidents, but never one over such a long distance.
"The way we explain it right now is some people get stuck in the middle of wakefulness and total sleep."
While parts of their brain that require a higher level of thinking could remain asleep - preventing them from discussing things such as philosophy or history - active lower brain centres could still see people walking around, and they were able to navigate. "It's like an elevator being stuck between two floors."
Dr Fernando said a common trigger for such behaviour was stress and, to a lesser extent, alcohol, sleep deprivation and medication effects.
Sleep Well Clinic director Dr Alex Bartle believed yesterday's incident was most likely a case of microsleeping - a very short episode of sleep usually caused by extreme fatigue.
He said there was evidence to suggest some types of sleeping tablets could lead to strange, automatic behaviour.
"In automatic behaviour, people can drive but they usually wouldn't go very far. Doing that sort of distance, she would most likely be awake, just not thinking straight, and definitely microsleeping."
But Professor Richard Jones of the NZ Brain Research Institute in Christchurch, who is studying the issue, disagreed, saying microsleepers were totally non-responsive.
"My take on it is that she's a severe sleep walker," he said. "The point is, she did drive safely 300km so she was clearly quite responsive. She could perceive the road conditions and she could respond to that. I've never heard anything quite like it. I'm quite intrigued by it."
Law professor Warren Brookbanks said there were a lot of different elements in this woman's case, but police would generally have to seek suspension of her licence through the courts.
"I would say the police would have good grounds for bringing some sort of prosecution. Obviously there are a lot of elements to it and it's a very unusual case."
The courts would then decide her fate, based most likely on whether they thought she knew she was impaired but drove anyway, or if the entire journey was completely involuntary, Professor Brookbanks said.
He was unaware of any cases in NZ of legal proceedings against a sleep-driver, but sleep-acts had been used as a defence in other cases.
In June, a man escaped punishment in Auckland District Court for indecently assaulting a sleeping girl because he suffered from a condition called "sexsomnia".
Sufferers carry out sexual acts in their sleep and later have no memory of the event.
- additional reporting: APN
