The Independent reports the accident happened at the Windsor Polo Grounds in Australia on April 2 after Weir's horse fell.
The model was rushed to a nearby hospital and had been on life support for several weeks, as per the outlet's report.
Tributes for the 23-year-old model have been pouring in online since the news came out.
Modelling agency Scoop Management shared a video of Weir's photos set to Coldplay's song Everglow.
"Forever in our hearts 🕊🤍" the caption reads.
Australian photographer Chris Dwyer posted a tribute to Weir on his Instagram story on Thursday.
"You were one of the kindest souls in the world, you lit up a room and the world is a lot darker now that you're gone," he wrote.
Chris Dwyer's tribute to Sienna Weir.@chrisdwyerphoto_ | Instagram
Model Sofia Reynal also posted a series of photos with Weir on her Instagram page.
"For the short time I was blessed to have met your kind loving, radiant beautiful soul I am so very grateful for," she wrote.
"My heart breaks, no words could ever describe the pain that stems from so much love."
Many others have also been posting their condolences on Weir's most recent Instagram photos.
"Taken from the world far too soon! RIP Sienna," one person commented.
"Rest easy beautiful angel ❤️" another person wrote.
Weir was one of 27 finalists in the 2022 Australian Miss Universe competition, as reported by news.au.
In a 2022 interview with Gold Coast Magazine, Weir talked about graduating with a double degree in English literature and psychology at Sydney University. She had also shared her hopes to move to London in the next few years to be closer to her sister and expand her "professional and social network."
In the interview, the Australian model also talked about her love of horse riding and show jumping.
"I’ve been horse-riding since I was 3 years old and can’t imagine my life without it," she said at the time.
