I've had wanker cops when it comes to both situations.
The rego, i was living out of home, with a mate because it was quicker to get to work. My rego ran out at midnight (was pulled up at 1am) and that still wasn't good enough for them. Went home straight away, payed on credit card, and back out driving again.
Because i was on my P's at the time, its classed as a breach of conditions, and automatic disqualification, plus the $400 fine or something. I think that was harsh considering my car was out of rego for all of about 2 hours, and i had a legitimate reason.
On the P plate thing, i say don't drive with either and hoe you don't get caught.
Years ago a cop pulled me over (well, i pulled into a servo, and he thought he needed to hold up peak hour traffic to do a u turn to get back to me). My car was on defect, with the sticker displayed, and permission to drive it. After going over my car finding nothing, he then sends out a fine for not displaying a P plate. Both where there, just the back one the corner had fallen down.
Obviously not having the time and money to fight them in court, i had to cop it, and he knew that. In the end, i drove around without them up (yeah blah blah its illegal to remove a defect sticker), and believe it or not, not once did i get pulled over. When i did i put the sticker back on, and showed the permission to drive it (4 random bretho's easter weekend, at least there doing there job)
I believe my old Sig that wasn't allowed on here comes to mind, said cops are tops with an extra word added at the front