A trans -Tasman police operation is cracking down on highway heavyweights.

The month long operation between New Zealand and Australian police's focusing on the safety of heavy vehicles travelling the country.

Operation Twistlock involves vehicle safety, fatigue, speed and substance use - with police setting up roadside patrols.

Road Policing Assistant Commissioner Dave Cliff says the industry's greatly improved over the last few years.

"It's just a matter of continuing to focus on particular safety issues. For example, today we found several heavy vehicles where the brakes aren't performing at the levels they should be.

"Vehicle defects, also looking at educating drivers about ensuring that they're not driving tired."