Ground contact width in relation to an axle: means the distance between the outermost point of ground contact of the outside tyres on each end of the axle.
Ground contact width in relation to an axle group: means the greatest ground contact width of all the axles in the group.
Figure: Illustration of ground contact width of an axle
For more definitions, see Vehicle Terms Explained or view the regulation: https://www.legislation.qld.gov.au/LEGISLTN/CURRENT/H/HeavyVehMDLNR.pdf