The 24-year-old Dapto woman was hang-gliding at Stanwell Park on Saturday when her boyfriend of 4½ years, Shaun Duncan, unfurled a giant banner saying "Erin will you marry me?" on the beach below.

"I was up in the air and my instructor said, 'Is that your crew down there with a sign?'," she said.

"When I eventually saw what it said I was just crying and sobbing."

Her surprised instructor Greg O'Brien quickly asked his passenger if her answer was going to be yes or no - just in case she needed to make a quick getaway at the other end of the beach.

"But I just said 'it's a yes, it's a yes'," she said.

Once landed, the bride-to-be ran about 100 metres up the beach to where Mr Duncan was standing.

Her fiance knelt on one knee and held out an engagement ring as other people at the beach, including about a dozen of the couple's friends and family, cheered and applauded.

"He said 'Will you marry me?' and I could tell he had a tear in his eyes," she said.

"I was still crying and I just grabbed him and kissed him and said yes."

Ms Richardson said the sky-high proposal came as a huge shock after Mr Duncan conducted a top-secret operation to get her to Stanwell Park without an inkling he was about to propose.

To make his giant sign, he bought two large sheets, stitched them together and covertly hung them on the clothes line to spray paint the message while Ms Richardson was at work.

He then hid the engagement ring and sign in the boot of his car well in advance.

"He said he'd been thinking about it for a while - he bought me the hang-gliding voucher last year for my birthday and he already had the ring - but it was a total surprise and he didn't even tell any of our friends or family until I took off," she said.

"I told him 'you really got me' and I really didn't think that he would."