Alex, I will assume you work for a Constitutional Corporation here..
Can someone be employed as a casual, without loading
This is unlikely, as your total salary will be made up of a base, casual loading, additional allowances if any, and super.
If you are covered by a Modern Award, the award will most likely stipulate the casual loading required. If you are not covered by a Modern Award, then the National Minimum Wage Order kicks in, providing a Casual Loading of 22%.
"accruing holidays against the future when they will be made permanent, i.e. after their probationary period of three months?"
Once again I don't understand how this would work, as the National Employment Standards prescribes Annual Leave to be accrued progressively, and is payable to the employee on their termination, regardless of time with employer ie. less than 3 months.
If so, what arrangements should be made for paying out holidays when they are terminated?
Alex you are either a casual employee or a permanent. If you are casual you should receive a casual loading (clearly indicated in your salary breakdown) and no leave entitlements.
If you are a permanent employee, you will accrue leave from day one, and any accrued leave should be cashed out when you are terminated from your place of employ.
I would recommend you get onto fairwork.gov.au and either give one of the operators a call, or get on their Live Chat, to clarify the above which I have just explained.
Hopefully our responses match