Roser, 63, is on trial for murder after Mr Saunders, 54, died while working on a property north of Brisbane in November 2017.
Sharon Graham, 61, asked Roser and another lover Peter Koenig to kill Mr Saunders and make it look like an accident in a bid to claim his $750,000 life insurance, the Crown alleges.
Police footage has been shown to the jury of Mr Saunders' legs protruding from the woodchipper at the Goomboorian property near Gympie after clearing trees with Roser and Koenig.
Police audio of the night officers first attended the property were played in Brisbane Supreme Court on Friday.
"Don't know what happened, it happened so quick, eh ... oh my God, I just can't believe it," Roser told Detective Sergeant Christopher Duhig.
Roser described Mr Saunders as a good little worker but claimed his friend had been mucking around near the chipper.
"All day we were into him about it. You just can't tell him, eh. You couldn't tell him anything," he said.
"Every time you looked around he was trying to shove things in the chipper and I was like 'no stay away from it, Pete and I can handle it'.
"It's a really dangerous machine ... it chews it up really quick."
Koenig has accused Roser of killing Mr Saunders by repeatedly hitting him with a metal bar before they carried the body to the chipper.
Koenig used a branch to push Mr Saunders into the machine but decided to stop with the legs still sticking out so it appeared to be an accident, the court has heard.
Roser told Det Sgt Duhig at the property that he didn't see what happened but turned around and saw Mr Saunders in the machine.
"I really can't work out what happened ...never heard a sound," he said.
"I tried to pull him out. I just couldn't get him out of there. He was stuck in there - I tried to save him."
Roser said in his police statement read in court that Mr Saunders looked inexperienced around a chipper and believed it was the first time he had used one.
"He seemed oblivious to the dangers. He would stand there and watch the branch disappear into the machine," he said.
"Sometimes I actually grabbed his shirt and pulled him away.
"He would laugh when I tried to teach him about safety."
Det Sgt Duhig said at that stage he was treating Mr Saunders' death as an industrial accident.
He said two days later he started receiving calls from people who knew Mr Saunders.
The investigation shifted toward following up that information and Graham was interviewed, he said.
By January 2018 police organised telephone intercepts for mobiles of Roser, Koenig and Graham.
They also looked at retrospective texts on the phones.
Listening devices and a camera were then set up at Graham's Nambour residence that she had shared with Mr Saunders.
A surveillance camera was also established at a neighbour's house.
The trial before Justice Martin Burns continues.
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