Nearly half the country's worst drink drivers are not being ordered by the courts to fit a vehicle immobiliser, despite this now supposedly being mandatory.

A report from the Automobile Association also shows that when the courts do impose the order, fewer than 50 per cent of offenders actually comply and get one installed.

A law change in July last year requires judges to make the worst recidivist drink drivers install an alcohol interlock device for at least a year once they regain their licence.

This technology stops someone being able to start their car without first blowing into a breathalyser to prove they're booze-free.

But the AA's report out today has found despite the law saying interlocks are mandatory, in 48 per cent of cases judges aren't requiring them to be installed.

For the AA's Dylan Thomsen that's not good enough.

"That's a huge concern to the AA, we want every high-risk drink driver who can get an interlock in their car to be getting one.

"It doesn't look like the approach that's being taken at the moment is working."

Judges have some discretion about imposing an interlock sentence, such as when an offender doesn't own a car or hold a driver's licence.

The Criminal Bar's Len Anderson says the cost of an immobiliser is also a factor.

"One of the problems is that effectively you need to be employed in order to be able to afford an alcohol interlock device.

"So for those who can't afford it, it's just a complete waste of time."

Dylan says even taking the exclusions allowed to judges into account, he still couldn't understand how the sentencing numbers were so low.

"For us it ... raises a lot of questions that we can't answer at this point.

"We've written to the Ministry of Justice and to the [Minister of Justice] requesting a meeting to talk about this."

The report also says of the 4200 interlock sentences handed down in the past year, fewer than 1800 people actually complied and got the units installed.

Gavin Foster from Smart Start Interlocks says his business installed about 30 immobilisers nationwide a week.

But he says that number should be much higher.

"[People who have been sentenced to have an immobiliser installed] just disappear into thin air if they want, no one ever tracks them or follows them up or anything.

"There's nothing there to enforce it."

Dylan says a government agency or ministry needed to step up and ensure people were complying with the law.

"What we need is someone to be tasked with that responsibility and ... actually have the list of all the people who do get sentenced - all their names, details - and then monitoring that as [offenders] go through the system."

Acting Chief District Court Judge John Walker says he's happy with the way judges are applying the law.

He says the AA's research suggested people who were in line for an interlock sentence were successfully arguing for an exemption.

He agreed cost was a likely barrier to getting an immobiliser installed and pointed out if people didn't get one put into their cars they remained disqualified to drive.