Response from Jeremy Bath (Chief Executive Officer of NCC)

CN has no authority to direct AP to deliver the mail. AP operates as a Government Business Enterprise governed by federal laws, including the Australian Postal Corporation Act 1989, which sets its commercial framework and public service duties.

You can make a complaint to the Postal Industry Ombudsman (PIO) via the link or call 1300 362 072. I've previously been advised that you must first complain directly to Australia Post (call 13 76 78 or visit their support page) and wait for a response. I'm sorry there appears no quick solution.
https://www.ombudsman.gov.au/complaints/postal-industry-complaints

The Australian Government is represented by two Shareholder Ministers, the Minister for Finance and the Minister for Communications, Urban Infrastructure, Cities and the Arts. So my advice would be to contact either of these ministers or try recontacting the office of the Federal Member for Newcastle.