Agree with the others who say you need a permit...
Any trailer sold in qld to non qld resident must have the registration removed/cancelled and a temporary movement permit issued.
Yes if its registered yes it can be driven anywhere by the owner or friend family etc..however the rules change once a sale/ transaction has taken place..although you wont get booked for unregistered vehicle you may find some complexed issues relating to liability insurance in the event of a crash and personal injury claims. if a transaction/sale has occurred.
if you kill/ injure someone including your passengers I guarantee the prev owner will be letting the whole world know it was no longer his trailer as it was after the sale, therefore the personal injury component of his QLD based rego would not apply as it is non transferable unless a fellow qld resident has purchased the trailer.
Safest way.
1- purchase the trailer unregistered have this listed on the receipt and have prev owner remove plates(they are responsible for returning them to qld transport)
2- speak with the SA roads authority about a permit (if as some say theirs covers multiple states then check with each state that they honour this)
3- OR Get a qld movement permit which normally has your final destination written on it..this permit has little to do with registration and a lot to do with personal injury insurance.
Do it this way and your safe and compliant.
many people do it the other way..(just take it drive it and return the plates later) and although it saves a few bucks it could cost you the biggest headache of your life IMHO.
Jet
Edit- I would be getting the permit from the state the trailer is registered in and have it cover me all the way home to my home state. Point A-B