Five months ago, Christopher Willemse was down on one knee at one of New Zealand's most picturesque spots, proposing to the love of his life.

This week he held her as she lay dying, gunned down while the pair should have been enjoying a music festival in Las Vegas.

Today Willemse shares his love story with the Herald, paying tribute to his slain fiancee Sandy Casey, who was one of at least 59 people killed in the worst mass shooting in US history.

Casey was killed when gunman Stephen Paddock, 64, opened fire on a crowd of more than 22,000 people attending the Route 91 Harvest Festival in Las Vegas on Monday afternoon (NZ time).

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The last photograph Willemse posted of his fiancee before she was gunned down in Las Vegas in the worst mass shooting in US history. Photo / Facebook
The last photograph Willemse posted of his fiancee before she was gunned down in Las Vegas in the worst mass shooting in US history. Photo / Facebook

Paddock, a high-stakes gambler who kept to himself, had holed himself up in a room on the 32nd floor of the Mandalay Bay Resort in the famous Nevada casino city.

He had a cache of guns and used them to kill the 59 and wound more than 525 others.

Police found 23 guns in his hotel room and another 19 at his home. He had modified two of them with a "bump stock", which in effect converted the semiautomatic firearms into fully automatic ones.

Casey and Willemse were at the festival with friends and near the stage when the gunfire started.

The 35-year-old was hit in the lower back and died moments later in Willemse's arms.

He tried to save her, sticking his finger into her gunshot wound to try stem the bleeding.

Then, he picked her up and started to carry her - dodging bullets as he attempted to get her to safety.

The Washington Post reported that when Casey stopped responding, Willemse said "I love you" and told her that she was "amazing".

Willemse later spoke to the Herald about his ordeal.

"The last moments were tough but she was strong and amazing as she always has been," he said.

"As far as coping goes, I believe I'm still in shock and haven't fully grasped the effect this tragedy will have on her family, my family ..."

Sandy Casey and Christopher Willemse at Giant's Causeway in Northern Ireland. Photo / Facebook
Sandy Casey and Christopher Willemse at Giant's Causeway in Northern Ireland. Photo / Facebook

Casey grew up in the town of Dorset, Vermont, and obtained her Bachelor's and Master's degrees at the College of St Joseph in Rutland, about an hour away.

For the last nine years she worked as a special education teacher at Manhattan Beach Middle School, in California.

Seven years ago she met Willemse, 32, when he joined the school staff as a behavioral therapist.

The pair worked together and reportedly bonded over their love of country music.

Three years ago they became a couple and came on a "dream holiday" to New Zealand in April this year.

The day before they were due to fly back to the US, Willemse proposed on the shore of Lake Wakatipu.

He told the Herald they were staying near the lakefront, just outside the centre of the southern tourist mecca.

Casey and Willemse cycling near their Redondo Beach, California, home. Photograph / Facebook
Casey and Willemse cycling near their Redondo Beach, California, home. Photograph / Facebook

"We woke up for breakfast on our last full day there and walked down towards the water," he recalled.

"Since the hotel was at the top of the hill, Sandy was not thrilled about walking down to the water but I made her do it anyway.

"She got to the water first and was admiring its sheer beauty and clarity.

"As she was getting her toes wet I had gotten on one knee behind her, waited for her to turn around, had the ring out and asked her to marry me."

The date was April 15.

"New Zealand was our favourite place on earth," Willemse said.

"We did many things in Queenstown but our favourite part of all was the jump off the Kawerau Bridge.

"(It was a) pure adrenaline rush.

"It was most definitely a dream holiday."

The couple spent 10 days in New Zealand.

They were planning on getting married on August 2, 2020.

Willemse and Casey share a drink in happier times. Photo / Facebook
Willemse and Casey share a drink in happier times. Photo / Facebook

They had been scoping out wedding venues and were excited about spending the rest of their lives together.

"Sandy was a true bright spot in this world that will now forever watch over her family and friends for the rest of our time down here," Willemse said.

He earlier told the Washington Post that Casey was "a kind soul".

"She was full of life and loved to live it," he said.

"She made everybody smile, she was an excellent teacher and loved the kids she taught.

"Everyone who meets her never forgets her."

After news of Casey's death started to spread her heartbroken fiance paid tribute to her on Facebook.

"As I sit and mourn such a beautiful life gone too fast, all I can say is look up and watch the birds fly high and free today as that's where I feel you smiling down upon all of us," Willemse wrote.

"I love you baby girl. Love you to pieces."

He thanked his family and friends for their "love and support" during "these trying times" and said it showed "how important this wonderful woman truly was".

"She lived life to the fullest and made me the happiest man in the world," he wrote.

A fundraising page has been set up to help Casey's family cover her funeral expenses.