The anniversary of Mrs Parnwell's transplant is always a time of mixed emotions.
"There is an amount of guilt. Someone has lost their loved one and I've had 20 years longer than they've had. It's a really hard concept to try and understand," she said.
"After 20 years it's still a really conflicting thing. Someone said I should use it as a second birthday but I don't feel like it. Often it's quite an emotional day for me. I don't think of me. I think of the donor and the donor's family and wonder who."
The last thing Mrs Parnwell, then 23, remembered was being admitted to Tauranga Hospital's intensive care unit after vomiting for four days.
Having got sunburnt at the weekend she thought it was a bad case of sunstroke but, unbeknown to her, she had contracted Hepatitis B and her liver was shutting down.
The next day, after losing consciousness, she was flown to Brisbane, as liver transplants were not performed in New Zealand at that time.
And on March 6, 1993, a week after becoming ill, she was given a new liver.
"I was young and healthy and overnight it happened and I woke up in Australia. It was such a shock, psychologically and physically. For a long time I almost resented it," Mrs Parnwell recalled.
"I have a letter from a doctor who said I was one of the sickest pre-transplant patients they had ever seen."
She stayed in hospital three months - and was given a $70,000 bill.
An appeal was launched in Tauranga for Tania McBride, as she was then known, and successfully raised $150,000 to pay hospital and other expenses, including the flight.
The Bay of Plenty Times covered the appeal at the time.
An architectural draftswoman at the time, it was two years before Mrs Parnwell returned to full-time work.
"It took a long time. I suffered with depression and I guess post-traumatic stress disorder and that's still a lurking thing," she said.
In 2000, she married Gavin.
Nine years ago they had their son, Riley, who was conceived just as they were about to try IVF.
While the anti-rejection medication she takes twice daily is a constant reminder of what she has been through, Mrs Parnwell said she enjoyed good health.
"I can't believe it's been 20 years. It's amazing."