OK, well the Queensland Government has been quite helpful in setting out what rules apply for Restricted Businesses https://www.health.qld.gov.au/system-governance/legislation/cho-public-health-directions-under-expanded-public-health-act-powers/business-activity-undertaking-direction#schedule1
Unfortunately what is or isn’t a Restricted Business is not so obvious (see the schedule in the directions above) and there is very little guidance for Non-restricted Businesses (essentially everything else).
There does not appear to be any direction for operators of non-restricted businesses – eg professional offices – allowing them to require proof of vaccination of customers. That said, those operators have duties of care to employees and patrons. There is a tension between those obligations and the entitlement of customers and clients to privacy, also risks in the way in which a face mask mandate is imposed https://www.qhrc.qld.gov.au/your-rights/covid-19-and-human-rights/face-mask-guide-for-business
What would be useful would be for a Public Health direction setting out a list of what an Essential Business can and cannot do, in one place, rather than running the risk of falling into a trap while having the best of intentions. One can only hope.