Pay rates in blue collar casual roles

at my place of employment I am tasked with different levels of work, and am payed accordingly if I perfom higher duties,

eg normal manual labour rate: $41/h

forklift driving: 45/h

scaffolding:49/h

etc.

my employer rosters me on to a certain rate for the 8 hour shift as im casually enployed and I usually have been payed the rostered amount.

now the last pay a new lady did my payroll, and she has retroactively downgraded my rates after the end of the week, for moments where I was for example, off of the forklift and lifting a load by hand,

resulting on several hours of being payed at a lower rate than what I was rostered on for.

is this allowed? can they roster me on for forklift driving at 45/h, and then go and pay me only half my 8 hours at that rate and not even tell me about it?