It is issued under the terms of a special piece of APEC security legislation and says that if a banned person is found within the declared area (how they might get in given the 2.8 metre fence is not discussed), they can be imprisoned.
Under the APEC Meeting (Police Powers) ACT 2007 (NSW) an arrested person can be held without bail until 12 September. The Act specifies a “presumption against bail”. This is a draconian piece of legislation, similar to banning orders in apartheid South Africa.
It seems that anyone is now a suspect in Sydney, as the Orstrahyun blog points out.