A New Zealand forensic biologist has cast doubt on the way key exhibits in the Claremont serial killings case were handled by WA pathology laboratory PathWest, in evidence given at the trial of Bradley Robert Edwards.

Key points: The New Zealand lab tested DNA samples taken from Ciara Glennon

The New Zealand lab tested DNA samples taken from Ciara Glennon No male DNA was detected on any samples taken from Ms Glennon's body

No male DNA was detected on any samples taken from Ms Glennon's body Female DNA was found on some of the control samples — which should be blank

Edwards, 51, is accused of the murder of three young women in Perth in 1996 and 1997 — Sarah Spiers, Jane Rimmer and Ciara Glennon.

The women had been out in the western Perth suburb of Claremont when they disappeared and while Ms Rimmer and Ms Glennon's bodies were discovered weeks after they vanished, 18-year-old Ms Spiers has never been found.

On Thursday, Sally Ann Harbison, a DNA expert from the Institute of Environmental Science and Research (ESR) in Auckland, told the WA Supreme Court a number of samples taken from Ms Glennon's body and from her home were sent to ESR in 2004 for analysis from PathWest.

DNA samples from Ciara Glennon's body and her home were tested by the NZ lab. ( ABC News )

These included fingernail clippings taken from Ms Glennon during her post-mortem examination.

No male DNA found on samples

Dr Harbison testified that male DNA was not detected by ESR on any of the samples taken from Ms Glennon's body. She also said no DNA was detected on some of the samples, while only the 27-year-old's own DNA had been found on others.

But four of 21 control samples sent from PathWest and analysed by ESR were found to contain female DNA.

Control samples should be blank — that is they should not contain any DNA, Dr Harbison said.

Bradley Robert Edwards denies killing the three women. ( Facebook )

Asked by defence counsel Paul Yovich SC how such contamination could have occurred, Dr Harbison said it was most likely they had been contaminated by PathWest, which had already conducted its own DNA extraction and testing on the samples.

Dr Harbison said ESR analysed more than 50,000 samples a year, and only about one or two per year on average were ever found to be contaminated.

But she said the contamination of the control samples had not affected ESR's essential finding that no male DNA could be detected on Ms Glennon's fingernail samples.

One of the samples tested at ESR was from Ms Glennon's left middle finger, known as AJM 42, which was to go on to play a crucial role in the prosecution's case.

Years pass before breakthrough result

While the NZ laboratory detected no male DNA in the sample, when AJM 42 was sent to the UK for more sophisticated testing four years later, it was combined with another sample, AJM 40, to produce a breakthrough result — the presence of trace amounts of male DNA.

But it was not until eight years later in 2016 that the DNA was matched to Edwards.

The former Telstra technician does not dispute that it was his DNA found on the fingernail samples, but contests how it came to be there.

Mr Yovich is trying to show that the fingernail samples could have been contaminated at some point during the lengthy and involved process of testing them.

But in an opening address to the trial, state prosecutor Carmel Barbagallo SC argued the DNA results obtained by both PathWest and ESR were "reliable" and the processes involved were "free from any evidence of contamination, whether specific to the exhibit or extracts being tested, or to the process within the laboratory generally".

Dr Harbison was also questioned about precautions taken by ESR against contamination and she confirmed that protective clothing and gloves were always worn.

PathWest scientist Aleks Bagdonavicius testified on Thursday afternoon about the way his laboratory handled crucial samples and exhibits.