I have found a clipping from the previous year that featured in The Sydney Morning Herald when the proposal was tabled with Sydney City Council which shows an even more extensive network of walkways than what has eventuated.
Source: Frizell, H. 1971. "New deal for hurried Harold". The Sydney Morning Herald, November 2: 9.
For instance, there would have been a second underground walkway under George Street near Martin Place. Australia Square was envisioned to be linked (it never did) through to Wynyard. Even the walkways under the Western Distributor were to form the network. Sadly the Western Distributor walkways were never connected. It was envisioned that the old tram tunnels under Wynyard would be a pedestrian underpass. As we know it is a parking station today. We also get a mention of a proposal to close Pitt Street from King to Park Streets between 12pm and 2pm. We did get a 24/7 pedestrian mall in Pitt Street (From King to Market Streets) in 1987.