It should make no difference that the bus companies are private. Transport for NSW collects all the income from tickets/trips (the fare box goes to TfNSW) they then pay the bus operators to run the services (based on hours and kms). The bus operator gets the same money regardless of if anybody is on the bus (although if route is always empty TFNSW may cut it)
It works the same for both STA (public bus operator with city routes) and the private operators
ticketing, planning, routes and assets are all controlled by TfNSW with the idea that they can change operators via tender
The special event buses (Olympic park etc) are similar transport pay the operator to run the bus, and again based on time and kms not number of people
Short answer is if the event has integrated ticketing (ie the event organiser has paid an agreed fee to TfNSW for “ free” transport) the operator should let you on it actually makes no difference to them if you tap on or not as they don’t keep the opal fare.