"His enthusiasm flowed out of him," said Mrs Hagar.
"I was sceptical - I told him, 'come down from cloud nine - we're never going to win anything.'
"But he said, 'Mum, we will.'
"My son did what many people never do - achieved his dream."
And the Hagars have exceeded Derek's expectations, coming away with the Bouchard-Finlayson trophy - awarded to the top pinot noir producers from around the globe - and a nomination for the IWSC New Zealand Wine Producer of the Year.
"Apparently, the judges were blown away by the pinot noir, and thought it was a French Burgundy," said Mr Hagar.
"The trophy is meant to be the Oscar of wine - it's a mammoth thing for a small vineyard in rural New Zealand.
"Derek would say, 'Dad, we're close to gold.'
"I wish he'd been here - he deserved it."
The Hagars bought Lansdowne Estate in 1998, and began putting down vines in the early 2000s.
The vineyard sits on the same land as the Beetham Estate, where Masterton pioneer William Beetham made international-standard wine in the 1880s, although the vines were ripped out during the Prohibition era.
Reviving a 19th century vineyard was hard work for the family - removing boulder-sized rocks from the soil to plant, replacing their first vines after they died and tending to all the crops by hand.
"We did everything the hard way - but Derek always said there was nowhere in the world he'd rather be than on the vineyard."
Derek's passion was further fuelled after a negative meeting with a Wellington sommelier while trying to pitch their wine.
"The guy said, 'come back in seven or eight years, when people know who you are,' said Mr Hagar. "That made him want to work harder.
"People have told us now we've got the trophy, we will have wine sellers from all over knocking on our door."
Following their win, the Hagars, whose wine is served at Iberia and The Copthorne, hope to put Masterton on the map as a viticultural hub.
"Masterton is so underrated," said Mr Hagar.
"But here, we've got the soil, the warm weather without the wind, the infrastructure and the labour force - it's perfect for the wine industry.
"We've already shown Masterton can take on the world's best and win."