Historical shipping records show no evidence of a passenger vessel named SS Britanis sailing from Southampton to Sydney in December 1973.
The SS Britanis (formerly SS Monterey) was active in the 1970s but primarily on North Atlantic and Caribbean routes. By late 1973, she was likely operating for Chandris Lines on Mediterranean or Atlantic cruises, not a liner voyage to Australia.
For passenger lists, check the National Archives (UK) (BT 27 series for outbound UK passengers) or the National Archives of Australia (inward passenger manifests). Specify the correct ship name and date.