UK to AUS Trcuk License. Really need help

You will need to contact the WA Dept of Transport regarding changing your license over but if I were to bet on the situation coming from another country despite it being Great Britain I would expect you would be required to undergo another road rules test and do a full on driver assessment for a heavy vehicle.

This is from a copy of the WA Dep of Transport : A person can drive on an overseas licence upon arrival in WA. The overseas licence must be valid and carried while driving. If the overseas license is a language other than English a translation of it into the English language must also be carried.
A person that holds a permanent visa granted under the Migration Act 1958 cannot drive in WA on their overseas license after three months. The three months starts once the person lives in WA and has a permanent visa. The three months does not start on the date the permanent visa was granted unless the person already lives in WA. After the three months, the person must hold a WA driver’s license to continue driving in WA.

You license would need to be equivalent to the Australian version to be able to drive Semi - trailers, or larger, e,g B- Doubles and Road train combinations.

Bottom line is you can drive on your overseas license the equivalent vehicle your licensed to drive from the UK but only for 3 months after this point you will need to change it to a WA license equivalent.