"The medication I take for my cancer leaves me with very little appetite. I can get to 11am and think 'Have I had breakfast?'.
"The long and short of it is it had been a very, very long day. It got to midnight and it was time for me to go home. I jumped into my car. It's probably the biggest mistake I've made."
McAlpine said the night had been an "emotional rollercoaster" because she had realised it could be the last time she had all of her good friends in one room.
"I was so emotional I had to leave. As I was driving I was crying for the first time since I'd been diagnosed. I was having a good, old sob to myself. Then I could see the booze stop way in the distance and I was thinking to myself 'I'm not drunk'. As soon as I blew and they said 'You've got alcohol in your breath' ... I was devastated," she said.
"It's not like I was running around with 10 bottles of Bollinger in my tummy. I can honestly say I would have had three glasses of champagne over that whole evening."
McAlpine was not sure if she would be eligible for a discharge without conviction, but said she hoped to hang on to her driver's licence.
"If I lose my licence, which the law states I should, I won't be able to get to my daughter [11-year-old Shannon] who lives on the North Shore. I won't be able to get to my medical appointments," she said.
"It's probably the lowest point of my life. It's just awful."
McAlpine recently had her ovaries removed at the recommendation of doctors who believed it would extend her life by as much as three years.
She had explained to the police and duty solicitor that a side-effect of her medication was that she had little desire to eat.
"Life is a funny thing. I've got to treat every day like it's my last. I want to wake up with a sore stomach because of laughing too hard the night before. That morning I woke up with the wrong type of sore stomach."