The NHVR runs the National Heavy Vehicle Accreditation Scheme (NHVAS) for mass, maintenance and fatigue management, approves electronic work diaries, and manages heavy vehicle road access, including the gazetted networks that determine where restricted-access vehicles can travel.
If your fleet runs anything over 4.5 tonnes outside WA and the NT, the NHVR is your regulator. Start with the heavy vehicle rules guide.
General information, not tax or legal advice.