DOT, If you continue on from Sect 3.3 that you quoted from the Privacy Act it appears that it doesn't apply. I've highlighted and underlined the sections that may be applicable.
3.3 An APP entity must not collect sensitive information about an individual unless:
(a) the individual consents to the collection of the information and:
(i) if the entity is an agency—the information is reasonably necessary for, or directly related to, one or more of the entity’s functions or activities; or
(ii) if the entity is an organisation—the information is reasonably necessary for one or more of the entity’s functions or activities; or
(b) subclause 3.4 applies in relation to the information.
3.4 This subclause applies in relation to sensitive information about an individual if:
(a) the collection of the information is required or authorised by or under an Australian law or a court/tribunal order; or
(b) a permitted general situation exists in relation to the collection of the information by the APP entity; or
(c) the APP entity is an organisation and a permitted health situation exists in relation to the collection of the information by the entity; or
(d) the APP entity is an enforcement body and the entity reasonably believes that:
(i) if the entity is the Immigration Department—the collection of the information is reasonably necessary for, or directly related to, one or more enforcement related activities conducted by, or on behalf of, the entity; or
(ii) otherwise—the collection of the information is reasonably necessary for, or directly related to, one or more of the entity’s functions or activities; or
(e) the APP entity is a non‑profit organisation and both of the following apply:
(i) the information relates to the activities of the organisation;
(ii) the information relates solely to the members of the organisation, or to individuals who have regular contact with the organisation in connection with its activities.
Note: For permitted general situation, see section 16A. For permitted health situation, see section 16B.
A JW is not a member of the WTBTS Sydney Branch but as to whether they have 'regular contact with the organization in connection with its activities' could possibly be argued. I would think that the WTBTS would argue that letters are regularly read out that have been sent by the Branch and most of the publications would come from there also.