What is interesting are the bits that you've left out:
Exception
Subsection (1) does not apply if:
(a) the making of the record or disclosure is authorised by section 43, 44, 45, 47, 48 or 49; or
(b) the making of the record or disclosure is in the course of the person's employment or service as an entrusted person; or
What like in the course of interviewing someone as part of their employment or service? 
(c) the making of the record or disclosure is required or authorised by or under a law of the Commonwealth, a State or a Territory; or
What like telling someone the reason they've been detained? 
(d) the making of the record or disclosure is required by an order or direction of a court or tribunal.
It is much more complex, but a quick review of those sections shows that they can disclouse information in the performance of their duties. It is strange that someone being investigated is unauthorised considering they have an "evidential burden" under the criminal code in relation to these exceptions. (also something from section 42)
Also the definition of Immigration and border protection information is worth considering.
"Immigration and Border Protection information" means information of any of the following kinds that was obtained by a person in the person's capacity as an entrusted person:
(a) information the disclosure of which would or could reasonably be expected to prejudice the security, defence or international relations of Australia;
(b) information the disclosure of which would or could reasonably be expected to prejudice the prevention, detection or investigation of, or the conduct of proceedings relating to, an offence or a contravention of a civil penalty provision;
(c) information the disclosure of which would or could reasonably be expected to prejudice the protection of public health, or endanger the life or safety of an individual or group of individuals;
(d) information the disclosure of which would or could reasonably be expected to found an action by a person (other than the Commonwealth) for breach of a duty of confidence;
(e) information the disclosure of which would or could reasonably be expected to cause competitive detriment to a person;
(f) information of a kind prescribed in an instrument under subsection (7).