What happens to personal plates when entitlement expires?

[marcusjames]

My father had a personal plate five years ago but due to illness forgot to extend the entitlement period, held on retention. He would now like to put it back onto his car.

What happens to the plate if he does not hold it on retention? Does he still own it? Will he have to pay the back-dated retention fees for the past five years before being able to put it on his car?

Thanks