It is in a privately owned shopping centre carpark.
I have not read all comments, so I am not sure if anyone has already answered it, the term "private car park" in a shopping centre is a public area, although privately owned by the shopping centre but it is not private property as the public have access to it, meaning anyone from the public can go and park there. The term "private parking" as in the sense of meaning "private" should be like a private property such as a house, farm etc....., therefore, the police have the authority to issue infringements in shopping centre parking areas (or similar areas). I don't recommend disputing this infringement on the basis of private property as you will certainly lose and pay the court fees as well.
From what you wrote, if you can show that the disabled sign was not clear or hidden and you did not see it, you may have a chance, go and take pictures of the sign.
Good luck.
Yes, I, once, got an infringement in a shopping centre car park. If you are interested in reading my story see:
NSW - Riding a Scooter in Car Park Without a Helmet?