What ethical issue? the airline does not charge fares based on aeroplane type. There is no surcharge for an A330

If the aerpoplane type was changed from a 330 to a 737 after purchase, it would be understandable to be upset if not realised and unable to cancel. Looking at the schedules, the overnight flight on a Saturday is scheduled as a 737 and, if booked on the QF site, could see it was a 737 at time of booking. The seat map would show a 737 layout if selecting a seat. If purchased and then realised at seat selection time , for example, the OP could have cancelled

QF has base fares for PER-SYD of $2959 . $3578 and $4351 according to Expert Flyer. If the cheapest fare for flight wanted was $4200 and a person chooses to purchase it, that is on them. It is expensive, but QF is able to sell at that price

The business class on the 737 the OP went on was the standard business class that QF uses on its 737s and the same that would have been offered on any route operated by the 737.

Fare comparison wise, the business fares today for SYD=MEL is as high as $1311. The distance for SYD-PER is 4.65 times that of SYD-MEL , making the SY-MEL equivalent at $6096 based on distance - most SYD-MEL flights are operated by 737s. If people will pay those fares, that is what will be charged