Miss Riekkola will get a 3-stone, princess cut, white gold diamond ring.
Mr Sims decided to enter the competition after seeing it advertised several times at a local mall. He was told a week ago that he was a winner.
"It's happened pretty quickly. After I entered I thought, 'Oh, I'm not gonna win it'. And then I got it and I thought, 'oh no'."
The couple have been together for the past four-and-a-half years after meeting in London in September 2007.
"I was over on my OE and we met there - we worked for the same hotel. I noticed her on reception a few times and I thought 'Oooh, I'll have to talk to her sometime'. She ended up moving over here with me."
The couple - both students at the University of Auckland - had discussed marriage but decided to wait until they graduated in two years.
"But I thought, 'Well, it's a pretty good opportunity'," he said. "At least now she's definitely not going to be expecting it. I'm going to surprise her."
Mr Sims said he would be waking his girlfriend up at the crack of dawn to take her to the billboard - on Lake Road in Takapuna - and would hide her cellphone in case someone beat him to the surprise.
"I'm just hoping she says yes."
Nine billboards will go up outside Westfield shopping centres around Auckland and another three in Christchurch, Wellington and Hamilton.
Other winning entries include: "Since you've been stubborn about getting those glasses, I've made your Valentine's card easier to read. Love you Sacha, Jayson" in Albany; and "Babe, after a shaky start to our relationship you're my green sticker all the way. Love you long time xxx" in Riccarton, Christchurch.
Other entries read: "Nek minute 2 children and an engagement ring ... can't wait to marry you Alex Kane"; and "Scott, even if your idea of fun is taking me to an electrical store, I still love you. Love Sarah".
The billboards will be put up early this morning and will remain on display until February 29.
