The number of people declared unfit to stand trial has more than tripled over the past eight years, a figure described as "scary" by the son of a murder victim.

"Everyone should be concerned," said Dean Newman, whose father Kevin Newman was fatally stabbed in central Henderson in 2005.

His attacker, Martin Robert Lyall, was charged with his murder, but was found unfit to stand trial and was remanded as a special patient.

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Dean Newman believes the rise in numbers of people declared unfit for trial showed the mental health system was struggling to cope.

"The outpatient services obviously just aren't coping with the numbers of people they're having to deal with, the population's only getting bigger," he said.

"The process of getting someone admitted as an inpatient seems onerous in the fact that it just doesn't happen," Newman said.

"It's just scary."

Ministry of Justice figures show the number of charges on which the defendant has been found unfit to stand trial has nearly quadrupled since 2010.

There was also a spike in numbers from 2016, with 427 charges, to last year, with 645 charges.

But those numbers can be misleading, as a small number of people may face many of the charges.

The number of people being found unfit to stand trial each year is smaller, but has still more than tripled, with 59 in 2010 and 193 last year.

While there are fewer people being acquitted by way of insanity, those numbers follow the same trend – the number of people acquitted last year was 36, up from 10 people acquitted in 2009.

Forensic psychiatrist Dr Justin Barry-Walsh doesn't believe the rise is due to more mentally unwell people entering the system - he said it could come down to more case law being available to judges.

That meant judges deciding whether someone was unfit for trial had more formal material to compare the case to, which may have helped establish a more solid threshold for what made a person mentally unfit.

"It may just be that we're getting better at recognising people that really are unable to adequately understand legal processes," he said.

About 500 people go before the courts each year for a judge to decide on unfitness, he said.

Barry-Walsh suspected the rise might take in more intellectually disabled people, as often people with other mental health issues that would otherwise make them unfit could be identified as needing compulsory treatment, and after receiving it, could go ahead with the court process.

"It's always important to know when we see this kind of trend happening what's sitting behind it, because there may be implications in terms of how we practise," he said.

Auckland University of Technology law professor Warren Brookbanks said the legislation around unfitness might need to be looked at.

"Our legislation hasn't been significantly reviewed since the current rules were enacted in 2003 so it's probably a case for having another close look at the way the law is working and particularly the way that we are dealing with people who are found unfit to stand trial."

Brookbanks was unsure what was behind the spike from 2016 to 2017, but said it could be related to an increase in drug use.

What makes a person unfit to stand trial?

A person can be found unfit to stand trial because of a current mental impairment such as intellectual disability, or mental disorder or mental illness such as dementia. They could be unable to:

• Make a plea to the court and understand what will happen if they plead guilty

• Understand what they are charged with or that they are in a court room

• Communicate with their lawyer and tell them what they want them to do with the case

If a person is found unfit to stand trial they may be ordered to receive treatment or care in a hospital, secure facility or in the community. They may also be discharged without a care or treatment order being made.