Mr Dumper, 46, a former Waiuku antiques dealers, was shot at point-blank range at the party to celebrate his daughter, Jessie Anderson, moving into her home at Clearlake.
Ms Anderson had moved in with her partner, whose second cousin, Forsythe, lived next door.
Mr Dumper's wife, Karyn, cried in court as she re-lived seeing her husband shot dead in front of her.
"One minute my life was perfect, the next I was living the worst nightmare you can imagine," she said.
"None of us suspected this to happen and I will always have awful memories of watching my darling Brent fall to the ground and how he looked at me for a split second, when he had fallen in front of me, with love in his eyes.
"And in that split second my darling was gone."
Mrs Dumper said she had returned to New Zealand after her husband's death because she lost her job after she "could never get out of bed [or] function to do normal everyday tasks".
"My life was ripped apart when Brent was taken. Nothing has ever been the same again," she said.
"Brent was the love of my life, my soulmate. We'd been together 22 years, since I was 17. He was my first love and we raised our children together. We had a great marriage and life together.
"All that was taken away on that Saturday."
Mrs Dumper said when the gun was pointed at her after her husband had been shot she thought she was going to be killed.
"I thought I was dead. I thought that was my last second of life, my last breath," she said.
She said any loud noise like a car backfiring or a balloon popping reminds her of the "horror and helplessness" on the day Mr Dumper was killed.
"There is not a day that goes by that I'm not reminded of the trauma of that Saturday," she said.
"The heartache and the sorrow and the gut-wrenching sadness ... every day is a struggle, every day I'm reminded."
The trial heard there were drugs and alcohol consumed at the party during the afternoon and evening.
There was evidence that Forsythe became increasingly aggressive throughout the party and challenged Mr Dumper to a fight more than once.
Later in the evening, Forsyth, 39, went to his house next door and returned with a shotgun.
He did not say anything before shooting Mr Dumper, the court was told.
A second man, Michael McMillan, was shot at but managed to push the shotgun away, though pellets put holes in his clothing and grazed his stomach.
"[Forsythe] then turned on Mr McMillan and deliberately shot. It was only the act of Mr McMillan pushing the gun away that prevented what could almost have been certain death," Crown prosecutor Dan Boyle told the court.
Forsythe has been found guilty of the attempted murder of Mr McMillan.
Defence barrister Callum Cassidy had asked jurors to assess whether the witnesses gave "reliable, truthful and accurate evidence about what happened".
