Justice Minister Rob Nicholson says his department is reviewing a Supreme Court of Canada ruling that struck down some changes to the Criminal Code dealing with drinking and driving.

Three years ago, the federal government amended the law, making it more difficult to fight charges of driving under the influence.

"Our government takes the issue of impaired driving very seriously. That is why we took action to toughen our laws and keep our communities safe," Nicholson said in a statement Friday.

Nicholson said those amendments were to ensure "only scientifically valid defences" could be used as evidence to refute a roadside breathalyzer test that showed a motorist to be above the legal blood-alcohol content limit of 0.08.

Under those new rules, anyone challenging a drunk-driving charge after testing over the legal limit on a roadside breathalyzer had to prove the machine malfunctioned or was misused, and that a high reading was the result.

Following that, defendants had to prove further that they wouldn't have been drunk given the amount of alcohol they consumed — the so-called "Carter" defence.

Lawyers argued this reverse onus violates Canadians' charter rights — and the principle of presumed innocence.

In a 5-2 decision, the Supreme Court partially agreed.

The court said Friday that once a defendant has proven a breathalyzer machine malfunctioned, the rest doesn't matter.

But the court ruled that the onus of proof remains on the defendant to prove the breathalyzer or test was faulty.

The ruling stems from a case involving a Quebec woman who had argued the new law violated her constitutional rights by barring her from presenting a Carter defence. The trial judge agreed sections of the law were unconstitutional but still convicted her.

The woman paid her fine, but Quebec Crown prosecutors appealed on the basis that the application of the law needed clarifying.

The court ruled Friday that requiring a defendant to both prove that a breathalyzer had malfunctioned or been misused and establish a causal connection to a high reading "constitutes a serious infringement of the right to be presumed innocent," an infringement that "cannot be justified in a democratic society."

In a related case, R. vs. Dineley, the court restored the acquittal of a man whose case was on adjournment when the law was changed to eliminate the Carter defence as an independent defence against a drunk-driving charge.

The court, in a split 4-3 ruling, upheld the constitutionality of the law on that point but decided the retroactive application of the legislation to his case substantially affected his rights.

Nicholson said in his statement he was pleased that Friday's ruling "has recognized the reliability of approved instruments and upheld in part the constitutionality of those important amendments.

"Previously, testimony about an accused individual’s drinking pattern prior to driving, combined with expert evidence, could be used to prove that if they consumed the amount they claimed, their blood alcohol content would have been below the legal limit. This defence was allowed even though there was no evidence that the approved instrument had malfunctioned.

"The sections of the Criminal Code found constitutional by the Supreme Court will protect Canadians by limiting this defence to only cases where there is also evidence of either equipment malfunction or operator error."