New Zealand courts are turning more to "hired gun" expert witnesses, and it could lead to more miscarriages of justice, a leading forensic scientist says.
An Auckland University study released this week looked into the treatment of expert witnesses in New Zealand.
It said criminal courts and the Family Court were becoming more reliant on them, but experts mistrusted the courts so much they were refusing to give evidence.
Head researcher and former Whangarei Crown prosecutor Emily Henderson and Auckland University's Fred Seymour interviewed five district court judges, 27 experts and seven lawyers on the issue.
The experts complained of "grossly unfair treatment by cross-examiners and deeply flawed and inadequate processes for evaluating their evidence", they found.
The lawyers saw "real issues" with the way expert evidence was handled and welcomed the idea of changing the system.
The two main problems lawyers found with experts were bias and reluctance to attend, the researchers said.
"One, for example, was critical of New Zealand experts' reluctance to criticise each other, while two - one a prosecutor, one with extensive prosecutorial experience - raised concerns about bias amongst a small but prominent group of defence experts," they said.
Another said there were some highly opinionated "super-charismatic" experts who could influence juries by their presentation and who were "incredibly difficult to deal with" in cross-examination.
One lawyer said the experts were "scared about their own liability, scared of being made a fool of, scared of participating in a system that doesn't strike them as rational".
"Our interviewees also identified a small number of 'hired guns' or activist expert witnesses, some local but several from overseas, who appear regularly in the courts (especially for the defence) and whom they believe are seriously unethical in the way they represent the scientific issues to the court," the study said.
Extensive demands on their time, challenges to their competence and fear of making a mistake were cited as reasons for their reluctance.
Several lawyers said testifying was the experts' civic duty and "recompense for their powerful and well-paid positions" in society.
Dr Anna Sandiford, a leading forensic scientist who worked as adviser to the defence in David Bain's 2009 acquittal, said the issue dented the quality of the legal system.
"Miscarriages of justice because of expert witnesses - I know it has happened here," she said.
"We know there are three contributors to miscarriages of justice.
"One is poor or inadequate police investigation, which is followed by how the prosecution presents its case, the second will be poor or inaccurate forensic science or expert witnesses, and the third will be a poor or inaccurate defence team."
If one or more of those characteristics occurred in a case the chance of a miscarriage of justice would increase.
Unless experts were checked routinely, hired guns would always be taken on.
"You're talking about the quality of expertise given in court and how the legal system manages experts - that's really important," she said.
Henderson and Seymour proposed several changes to the system that could be implemented "immediately".
These included expert training for court, which would teach people what to require from the lawyer calling the witness, evidence law, and cross-examination tactics and rebuttal strategies.
Lawyers should also have training for how to assess expert evidence and how to brief experts, they said.
Sandiford agreed with all of the measures.