The HVNL covers vehicle standards, mass and dimension limits, fatigue management, accreditation and Chain of Responsibility. It is administered by the NHVR. WA and the NT run their own heavy vehicle rules, which matters for fleets that cross those borders.
For most fleet operators the HVNL shows up as duties: keep vehicles roadworthy, manage driver fatigue, respect mass limits, and be able to prove all three. The heavy vehicle rules guide walks through them.
General information, not tax or legal advice.