The Copyright Act contains provisions regarding the term of copyright; some of the key rules are set out in How long does copyright last?, which you can download as a PDF from this page.
Once copyright in an item expires, there are no longer any copyright-related restrictions on its use. This is sometimes referred to as being in the public domain. That said, the Library may restrict certain uses of public domain materials for other reasons, such as donor restrictions or fragility.
Calculating the copyright term for a given work can be complicated because copyright legislation has changed over time. For instance, the current statute, the Copyright Act 1968, came into force on 1 May 1969. It contains transitional provisions dealing with works already in existence prior to its commencement, and in some cases there are (or have been) different rules for the copyright terms of such items. Furthermore, it may be necessary to look directly to previous copyright statutes to work out whether older items are still protected by copyright. Thus, if you are dealing with older works (particularly those created or published prior to 1 May 1969), you may need to consider transitional and other provisions.