Mrs Leak told the court Mrs Lundy and her daughter's killer would have been "substantially spattered with blood, bone and brain tissue".
That could have been transferred to surfaces brushed up against by the killer, such as a car, Mrs Leak said.
She also said an explanation for paint chips being found in Mrs Lundy's hair, the same colours as Lundy's tools, was that she could have gone to the shed and perhaps ended up with flakes on her hands that she could have run through her hair.
"I wouldn't rule out that the paint has come from a weapon, but you have a problem here because you don't have the weapon to compare."
Under cross examination by prosecution lawyer Philip Morgan, she said she came to that opinion after last week hearing evidence from the family cleaner talk about the home being redecorated.
"I'd expect to find fragments within the garage from those tools and if she's touched surfaces with those fragments on them, then yeah, I think that could happen."
Earlier today, Mrs Leak told the court that officers coming and going from the Lundy household during their crime scene examination could have contaminated or lost evidence.
She said the killer could have left from a conservatory sliding door, which was used by officers as they moved in and out of the Palmerston North house.
It was a good location for "forensic interest" as the killer could have left fibres or fingerprints there, she said.
Police should also have been wearing full protective gear as soon as entering the property, rather than changing into the gear while on the property but before entering the house.
Defence lawyer David Hislop asked if a smear of Mrs Lundy's blood found on an open conservatory window could have been transferred there accidentally.
"I don't think anyone can rule that out," Mrs Leak said.
She also criticised the use of fabric overalls as protective wear, which were more inclined to pick up trace material.
Mrs Leak said some of Mrs Lundy's brain tissue could have stuck in blood that was spattered across her bedroom.
Mr Hislop asked if some could have stuck to the clothing of Mrs Lundy's brother Glenn Weggery, who was first at the scene.
"What I can't rule out is that some of the spatter could have had brain matter of Christine's and that's been moved on contact," Mrs Leak said.
Mr Hislop suggested that when Mr Weggery travelled in a police car, the evidence could have been transferred to another officer who examined Lundy's vehicle.
Mrs Leak said she could not rule out contamination at the scenes.
Earlier today, ESR forensic scientist Bjorn Sutherland said there was a low chance of Mrs Lundy's bodily tissue being accidentally transferred from the area near Amber, because he had not recorded tissue in the vicinity.
The trial in front of Justice Simon France continues.
