Ruby Hill is told nearly every day how great she looks because she's lost so much weight.

Yet, she has a rare and debilitating digestive condition that is killing her.

For the last three years, the 22-year-old has struggled to get food down without throwing up and now weights just 42kg.

She relies on a feeding tube to keep her alive but even that holds the threat of one day killing her.

Her questions are endless and the answers she is given are limited. There is no cure and few doctors have heard of her disease.

Hill's story started when she was just 19 years old, training to be a pilot in Ardmore. Healthy, fit and "living life to the fullest" until one day she was sitting around a table eating lunch and couldn't stomach it.

She was admitted to Auckland Hospital in 2015 with suspected early-onset appendicitis. She had surgery to remove her appendix and a ruptured cyst on her ovary.

But after surgery, the agony continued for several months.

Ruby's weight quickly plummeted from 72kgs to 42kgs. (Photo / Supplied)

"I suffered from severe pain and vomiting, rejecting everything that got ingested whether it was solid or fluid, small or large, it didn't matter.

"I would go back to the hospital complaining, only to be sent home with no tests and a bag full of laxatives, [the doctors] saying I was severely constipated," Hill said.

For the next nine months, Hill jumped from doctor to doctor until finally she was diagnosed with gastroparesis - a condition where the stomach is unable to empty itself of food properly.

"The constant nausea and vomiting, the feeling of that steady, dull nausea is awful. I can't eat what I want. Food makes me sick. I have to drain my stomach acid out of my tube so I feel less nauseous."

Hill was told there is no cure and it is unknown how long she will survive.

"My heart dropped at the word incurable. You think, 'what does that mean for my life?' And they just don't have any answers to give you. It's destroying."

Hill said when she was first admitted she weighed about 72kg but quickly dropped to 50kg before she declined to her all-time-low of 42kg.

"I remember when I found out. It seemed like just the other day I had passed my medical test for pilot training and everything was normal."

A spokeswoman from the New Zealand Organisation of Rare Disorders said gastroparesis was a relatively uncommon disorder which could be debilitating for those who suffered from it.

"Gastroparesis can have a significant impact on quality of life including the physical, emotional, and financial aspects of life.

"Sufferers may find that the nausea, discomfort, and pain associated with gastroparesis interferes with their ability to work, socialise, and maintain normal eating patterns," the spokeswoman said.

But for now, Hill said she was "stable enough" thanks to a feeding tube called TPN (total parental nutrition).

Ruby's little cousin cheering her up. (Photo / Supplied)

"Eventually it will lead to liver failure and I'll die. And that's not to mention a number of occasions where something has gone wrong with my TPN and my life very quickly has been on the line."

For Hill, every day is a battle.

"Most days I spend with a vomiting bucket or hugging a toilet. One minute you have no appetite then suddenly you get cravings like a pregnant woman but you can't eat.

"I can eat some things in small doses. Like some days I can do an egg on a piece of thin bread but I can't do fibre and that's what I crave for healthiness - fruit, vege, nuts I can't touch."

She said the hardest part was being misunderstood.

"I get told on a daily basis how great I look and that I've lost so much weight but yet I'm dying and they don't know that."

She said when she tells people she's got gastroparesis, they say "it can't be that bad, you can't die from it" but little do they know you can.

"Believe me I've been close. They don't quite understand ... and that's purely because it's a rare disease few people know about.

"After being diagnosed with gastroparesis I still get labelled as neurotic. So many doctors have never heard of the disease and I have to teach them before they can help treat me," Hill said.

Hill lives in hope of finding a cure before it's too late.

Ruby at her lowest weight of 42kgs. (Photo / Supplied)