Jamie Aaron Venn and Michelle Gayle Morcom were shot dead in a murder-suicide in Hargrave Cres. about 1am on Monday.
Mr Venn's father Trevor believes the gun used was once part of his collection.
"The family was going up to Sydney in 1996 and I had a single- shot (shotgun) so I left it with my brother," he said.
Trevor said his brother had then registered the gun.
"A mate had borrowed it and shot a kangaroo. He'd hit it over the head and shattered the stock. It was useless.
"Two years ago I got the gun back, but it was rusted and in pieces. It wasn't worth getting repaired so I wrapped it up and was going to make a novelty out of it.
"I'd planned to back it on some timber and put it on the wall somewhere with the words: `Gun, killed by roo,' as a joke."
The plan never came to fruition, however, and the gun pieces were stored in a locked shed.
Out of sight, they were eventually forgotten by Trevor.
"When the cops came round they told me the barrel was registered in my brother's name.
"I took them straight to the gun cabinet and showed them, `Look they're all there.'
"I'd forgotten all about the broken gun in the shed.
"For someone to fix it, they would have to have a fair bit of knowledge about guns and my boy didn't.
"Now I'm trying to cope with the fact that the barrel at least is the barrel of my old gun.
"It's not an easy thing to do."