The phrase 'in trade or commerce' is defined in section 2(1) of the ACL. The phrase includes "any business or professional activity (whether or not carried on for profit)". It covers trade or commerce within Australia, as well as trade or commerce between Australia and places outside of Australia.
This definition is consistent with what the courts have said about the meaning of the phrase, that conduct takes place 'in trade or commerce' when it involves selling or dealing, particularly for business purposes or as a livelihood.
When buying, selling or dealing is done by persons for their private purposes, this is not activity 'in trade or commerce'. For example, if an individual occasionally sells a car because they no longer need it, or because they want to replace it, this is not a commercial or trading transaction. But if a motor car dealer sells a car, the transaction clearly takes place 'in trade or commerce' because it is part of their business.
Many of the provisions of the ACL only apply to conduct that takes place 'in trade or commerce'. You need to be on the lookout for this phrase in the provisions you study.