The singer went on to reveal that no surgery was necessary, but that she will be living in a neck brace for the next four months as her neck hopefully heals itself.
"i am extremely lucky to be alive and/or not quadriplegic as it JUST missed my spinal chord..," the sister to Cody Simpson wrote.
"the way I see it, four months is a drop in the ocean when it comes to the rest of my happy & healthy life.. as you can imagine, I have been in endless tears and thanking every guardian angel watching over me in the pool that night plus every moment since ❤️💋."
The star continued: "to say the least - I have a new lease on life & will be forever grateful that it was not worse!!"
Alli ended the post by thanking her friends and family for their support during this rough time, as well as commending the paramedics and medical professionals for their incredible efforts.
"happy new year everyone - sending love & light 💡💛 hoping the start to your year has been a whole lot better than mine!! stay safe this year fam & DON’T DIVE INTO ANYTHING WHEN YOU DON’T KNOW IT’S DEPTH!!! 🤍🤍🤍 much love, Alli x."
The model was soon inundated with messages of support from many famous faces.
Among the well wishes was Alli's former I'm A Celebrity... Get Me Out Here! screen pal Dr Chris Brown, who wrote: "Wow. Thinking of you. Just getting all the bad stuff out of the way nice and early. Stay strong 💪."
Cameron Daddo also penned his support, adding: "Sending you love and 💯 health and happiness. Xoxox."
Bachelor and I'm A Celeb alum Abbie Chatfield added her voice to the mix, writing: "Sending you love angel."
Jackie 'O', who shared a screen with Alli on The Masked Singer, also commented: "Oh honey, you poor thing. Thankfully you are going to be ok. Sending lots of love ❤️."