"I'll never stop," she wrote. "You're in my heart forever. Wait for me."
Mulcahy said she asked her partner to pull down the blackout blinds in the home they had shared since January, when they relocated to the UK, and she received one final answer that made her fear the worst.
"His reply was 'will do' and then he stopped responding but before that we were talking about how we were going to go to London in three weeks," Mulcahy told Nine Newspapers.
'We just adore each other, we have not had problems in our relationship and I would be truly honest if we did."
My dude, my guy, my soulmate, my Joseph. Empathetic, creative, funny, generous, humble, kind, clever, protective,...
Mulcahy reported Day's disappearance to police, and had appealed for help on Facebook, with her heartbreaking post being shared thousands of times.
"If you have any information at all, if you've seen him anywhere, please reach out and let me or the Avon and Somerset police know," she wrote.
"This is extremely out of character for him, we always keep in touch every minute of the day. "Everyone who knows Joe loves him very much, he is incredibly special to me, and his family and friends. Please help and share so we can find him."
It is understood police did not launch a formal appeal until Monday, 24 hours after Mulcahy reported Day was missing.
Then, on Wednesday morning, Avon and Somerset Police confirmed the worst.
"In the early hours of this morning, police located and recovered the body of a man in Avon Gorge, Bristol," they said.
"While formal identification has not yet taken place, we have updated the family of missing 31-year-old Joseph Day.
"Our hearts go out to them and we thank them for their patience while inquiries into the circumstances of death continue.
"We'd ask the public to avoid speculation and respect the privacy of Joseph's family at this time."
Mulcahy's friends had travelled from London to help search for Day, and the couple's frantic parents had travelled to the UK from New Zealand to assist.
The couple had planned to return to New Zealand to marry at the end of the year.