AFAIK OZ and the UK do have a common defence treaty. In that the treaty that underlay SEATO, the Southeast Asia Collective Defense Treaty, is (IIRC) still in force for those countries that didn't formally withdraw or cease to exist (Pakistan and South Vietnam). Its affects in relation to the PRC depend on whether the signatories consider the CCP is actually Communist
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Article IV
1. Each Party recognizes that aggression by means of armed attack treaty area against any of the Parties or against any State or territory which Parties by unanimous agreement may hereafter designate, would endanger own peace and safety, and agrees that it will in that event act to meet the common danger in accordance with its constitutional processes. Measures taken this paragraph shall be immediately reported to the Security Council of United Nations.
2. If, in the opinion of any of the Parties, the inviolability or the integrity the territory or the sovereignty or political independence of any Party in treaty area or of any other State or territory to which the provisions of paragraph 1 of this Article from time to time apply is threatened in any way other than armed attack or is affected or threatened by any fact or situation which endanger the peace of the area, the Parties shall consult immediately in order agree on the measures which should be taken for the common defense.
The USA has a particular interpretation of IV.1
Article XI
(...)
UNDERSTANDING OF THE UNITED STATES OF AMERICA
The United States of America in executing the present Treaty does so with the understanding that its recognition of the effect of aggression and armed attack and its agreement with reference thereto in Article IV, paragraph 1, apply only to communist aggression but affirms that in the event of other aggression or armed attack it will consult under the provisions of Article IV, paragraph 2.
The treaty area's northern limit is just south of Taiwan & Hong Kong so way south of Japan.
Article VIII
As used in this Treaty, the "treaty area" is the general area of Southeast Asia, including also the entire territories of the Asian Parties, and the general area of the Southwest Pacific not including the Pacific area north of 21 degrees 30 minutes north latitude. The Parties may, by unanimous agreement, amend this Article to include within the treaty area the territory of any State acceding to this Treaty in accordance with Article VII or otherwise to change the treaty area.