IT took three years, thousands of dollars and a custom-built iPhone app, but Sahba Idelkhani finally got to propose the way he wanted to.

When Mr Idelkhani, a 26-year-old computer security engineer, wanted to ask for the hand of his girlfriend Melody, he knew the moment had to be special.

It turned out to be that, and quite a bit more.

In what looks to be the country's most hi-tech marriage proposal, Mr Idelkhani led his lover on a 200km scavenger hunt controlled by a custom-made iPhone app.

It ended with a catered dinner in a hotel penthouse and a surprise three years in the making.

But let's go back to the start.

"I wanted to do something a little bit different that hadn't been done before," Mr Idelkhani told news.com.au.

"I've heard of proposals with people utilising different things like aeroplanes and so forth, but I wanted to incorporate a little bit of technology.

"We're relatively young, we do have a fair bit of technology around us in our lives. She's got an iPhone, I've got an iPhone."

With that in mind, the Sydney resident set about designing an iPhone game that would lead his girlfriend on a nostalgic journey across the city visiting places special to their relationship.

Once he'd come up with the design for the game, Mr Idelkhani posted the job of building it on outsourcing site Freelancer.com.au.

The first quote came back at $750, but Mr Idelkhani decided to pay twice that to ensure the job was done on time. After finding a game developer in Texas, an early version of the app came back in five days.

Then, after he'd plugged in the right GPS locations, it was time to start.

"She was given the iPhone app on the day. I went to her and said: 'You've got an app, just follow the instructions'," he said.

The app pointed Melody in the direction of special locations on a map. When she got within 25m of the location, a message appeared.

"Once she was happy with the actual message, she had to take a photo as evidence (she was there)," Mr Idelkhani said.

While Melody was tracking down the locations, the app was also posting her progress to Facebook.

"Throughout the day when she was walking around and going to all these different locations it would constantly poll back to Facebook and share the images and the title of the level she was on," Mr Idelkhani said.

"So people were constantly being fed information about where she was."

The app led the couple on a journey across Sydney to favourite cafes, parks and train stations.

"I tried to optimise it so we didn't have to keep going back and forth, but considering we went to like, I think it was 12 different places, there was still a bit of travel," Mr Idelkhani said.

"We pretty much travelled, I'd say, over 200km in the car."

At the end of the hunt was a catered dinner in the penthouse of the Meriton serviced apartments in the CBD.

But that wasn't the end.

At the start of the day, Melody had been asked to bring along a few items, and she was given several others during the hunt.

"The final level (of the game) prompted her to go into the master bedroom, saying there was something in there for her," Mr Idelkhani said.

Just like in a video game, the app told her to use the items she had at hand to find her reward.

"She emptied things out and found that there was a little tool to open stitches," Mr Idelkhani said.

"She realised she had two teddy bears, so she opened one.

"Inside it was a note that I put in there three years ago, which was a message saying: 'Will you marry me?'"

Hang on. Three years ago? Yep.

"I gave it to her within the first six months of when we started formally dating. We'd been friends for about five years before that," Mr Idelkhani said.

"I was confident that she was the one so I planted that in there just in case. I was pretty confident it would happen, but if it didn't it would be hidden in there forever anyway.

"I doubt she would have found it."

But everything did go to plan, and now the couple are planning their wedding.

And what's on the agenda for that event? After the thrill of the proposal, it will be back to basics, Mr Idelkhani said.

"Nothing as intricate this time. We're having an outdoor wedding on a private property."