Believing they'd never see it again, the couple abandoned the search and bought a replacement ring.
Life went on and the Sangsters were kept busy raising five children.
In 1997 they moved to Whangamata but made regular visits back to Glendowie to visit friends and family.
Then, one Friday afternoon recently, they received a phone call they could barely believe.
It was from the South African owner of their former home, calling to say that she had been digging in the area outside the kitchen window and had found a gold and diamond ring. It was Val's missing engagement ring.
"The woman actually hit it with the spade and after being in the ground all that time it had just about disintegrated," says Bob. "But the two little diamonds were still on it."
And Val's reaction?
"She burst into tears. She just couldn't believe it and neither could I. We had spent so much time looking for it and all that time it was in the garden.
"When the lady found the ring she showed it to her neighbour who reckoned it must have belonged to a child because it was so worn away. That didn't seem right because it was gold, so she phoned us."
The Sangsters eagerly drove up to Auckland to collect the ring, and while it was too damaged and worn to wear, they decided to have its diamonds set on the replacement they had bought all those years ago.
That work is now being done and Bob is hoping to have it back from the jeweller in time for Val's 80th birthday on April 25.
"It's also coming up to 58 years since we were married."
The Sangsters now have 12 grandchildren and three great-grandchildren - "and another on the way," says Bob.
And while they wait for Val's engagement ring to get back from the jeweller's, they're both looking forward to another big event. At the end of the month they're travelling to Fiji attend the wedding of one of their grandsons.