"I can truthfully say, I have found my soul mate...I love you my fiancé...Mo ake tonu ake...."
Lou shared her own photo with a very similar caption, except it began: "With you I always knew... I said YES!!
Stan's post was flooded with congratulatory messages.
"Congrats Stan!! So happy for you both," X Factor star Dami Im commented.
"Woooooooooooo!!! Congratulations!! MakingMahiMatter on a Monday my brother," penned another follower.
"Yaaaaaaaaaaay!!!!! Congratulations guys," exclaimed another.
The couple have reportedly been dating since November 2013.
The happy news comes three years after Stan was tragically diagnosed with stomach cancer - an inherited mutation of the CDH1 gene.
The New Zealand singer had his stomach removed and doctors discovered not one fast-growing stomach tumour, but 13.
“I never cried once, I never felt sorry for myself, I wasn’t scared of what it would do,” he told New Idea.
“The cancer came at the end of a very long season of just losing everything – my passion, my joy in the industry, my money. I was just really unhappy in my whole life, and losing my health was the last part.”
Stan won the seventh and final season of Australian Idol in 2009, beating runner-up Hayley Warner.
"To be honest, I felt like I had a perfect experience," he told Mamamia of his experience.
"It was incredible, it's such a fun experience. I think if you're going in there with an open heart and be prepared to work - I think there's a big section of people who go in there thinking 'we're on a show, we're entitled, I'm a big superstar'. Nah, bro. You gotta work your arse off... You have to work harder than your gift."
This article originally appeared on New Idea.