In Commonwealth Acts all notes including margin notes, headnotes and footnotes (but not endnotes) are considered part of the Act and maybe used in interpretation.
In NSW, Vic, QLD, SA, WA, ACT, Tas and NT Acts, margin notes, headnotes, footnotes and endnotes are NOT considered part of the Act but may be used as part of the extrinsic materials of the Act.
Rhys's Statutory Interpretation Site 