"It's been an amazing day. A huge day. Pretty much everything has changed."
Mr Sims said he woke Miss Riekkola up early, saying he would take her to Takapuna for a cafe breakfast.
He walked her across a park to the foot of the billboard bearing his proposal.
As soon as Miss Riekkola read the message, she was overcome by emotion, he said.
"There was just absolute shock on her face. She had her hand over her mouth in shock. Then she said yes."
The couple didn't have long to adjust to their engagement.
They had breakfast together before Mr Sims went to start his first day at a new job.
Miss Riekkola stayed home to study for a three-hour exam she has to sit this afternoon.
Mr Sims said they were looking forward to celebrating properly this evening.
He and Miss Riekkola have been together for the past four-and-a-half years after meeting in London in September 2007.
They had discussed marriage but decided to wait until they graduated from the University of Auckland in two years.
That changed when Mr Sims decided to enter the Westfield competition after seeing it advertised several times at a local mall. He was told a week ago that he was a winner.
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.
