How can one day Dad be fine and the next a mess?

Yes, do keep an eye on any infections.

In this case I'm betting on infections. But you should also be aware that the kind of dementia associated with Parkinson's includes delusions and even hallucinations, and they appear early in the progression of the disease. I'm sure this is something the doctors will discuss with you if it seems applicable.

Thank you, will ask the nurses tonight to check

The UTI may be back. My mom had seven of them in nine months. Have him checked again.

He has now been transferred to a public hospital as the rehab hospital couldn't handle him. We did find out that he had a UTI and Chest Infection however these have now cleared after taking antibotics, this was over 4 weeks ago however his delusional thoughts are still there. There are times when he has been aggressive and un-cooperative. Prior to going into rehab he had no delusional thoughts whatsoever

Side effects of a new medication?

Parkinson's can be like that, especially when there is a UTI, (urinary tract infection.) How old is he, was he in the hospital or home prior to this?
He can rally. Is he getting physical therapy in rehab? Is he cooperative?

Have him checked for a urinary tract infection that can cause delerium and increased confusion in the elderly. Also just being in a strange place will also cause the problems you are seeing.