UPDATED 7.28pm:  British forensic expert says police and other investigators compromised part of the scene at the Lundy double homicide.

Lundy’s facing a retrial for the murders of his wife and daughter in August in 2000.
Defence witness Gillian Leak has been called early in the trial, as she has commitments in the UK she needs to get back to.

Today jurors in the High Court in Wellington heard how a single sliding door was found open by the first person on the scene, was then used as the main point of entry for those investigating the murder.

Leak says that shouldn’t have been the case, as it’s likely the assailant exited from that door.

“There's a good chance that whoever committed the offence will have made contact with that region and left behind something. Anything - footwear marks, fingerprints, fibres, DNA," she says.

Leak has also addressed questions from defence lawyer David Hislop, about the possibility of blood or tissue being taken from the murder scene to other areas of evidence.

She says it is possible that Christine Lundy’s brain tissue could’ve been transferred onto her brother Glenn Weggery, who was the first to find the bodies of Mrs Lundy and her daughter Amber.

Leak says it's also possible blood found on a window at the Lundy home that has signs of a forced entry, could’ve been a sign of contamination.

"I don’t think we can rule that out – to have fabric scene suits in that region after being in that scene, we can’t rule out [contamination] as a potential explanation for it."

Under cross examination from prosecutor Philip Morgan, Leak was accused of using speculation in her evidence when referring to paint flecks found in Christine Lundy's hair after the murders.

The Crown alleges the blue and orange flakes, which match tools found in Lundy's shed, came from the murder weapon.

However Leak says without knowing how many flecks of paint were in the house the evidence is neutral, as the flecks could've come in on shoes or clothing, or still be in the house after renovations.

Philip Morgan asked if it was "real speculation that somehow paint has got off tools in the garage, somehow got onto something she’s (Mrs Lundy's) touched in the garage, and somehow got on to her hands in the garage then got from her hands to her hair?"

Mr Morgan says the flakes then somehow had to get near a dead Amber Lundy, and into the hallway.

Gillian Leak says without the murder weapon it’s not clear how they got there, and those flakes could’ve come from the garage into Christine Lundy’s hair.