If it is private land and the driver is using the church or school, then it is not illegal per se. Inconvenient yes, illegal unlikely, dangerous not sure.
I'd be putting a letter under the wiper saying their permission to use the car park will be withdrawn if they can continue parking the way the do. Then take a photo of letter under the wiper.
If poor parking continues, another letter, another photo, and withdraw permission with a warning along the lines of that the driver may be charged with civil and or criminal trespass if they park on the property again. You can also write to the NSW equivalent of VicRoads and ask for registration details. You may have to justify why you want the details, then send a copy of the letter by registered post to the address on car registration. Not sure if the NSW dept allows this so worth a call first.