* People should only wear one set of medals and they should be directly related to their family, for example should have belonged to a brother or sister, dad or mum, grandfather or grandmother.
* In all cases these are worn on the right chest
* Only service medals and decorations mounted on a medal bar (full-size or miniature) can be worn by a relative. It is ok for people to wear miniature medals mounted on a medal bar as the weight is far easier to handle.
* Royal Honours insignia such as neck badges, sashes, sash badges or breast stars can't be worn by anyone other than the original recipient. The same rule applies to any Unit and Personal Commendations that the deceased wore on their right chest.
* The occasions on which wearing of relatives medals is permitted are limited to Anzac Day and Remembrance Day (November 11). In addition, it may be appropriate for next-of-kin and other relatives to wear relatives' medals on an occasion where either the relative's service or the unit in which they served is being commemorated.
For more details see the Defence site section on medals here.