FTDK
Setting of QNH was one of the marvellous
changes introduced by Dick and his NAS ****.
You should be using your departure QNH for the first 100NM. I work Approach and never give Area QNH to departures - even low level (A010 etc) - (except when specifically asked - and that's very rare now)
Try this for an explanation
http://www.dotars.gov.au/aviation/airspace_reform/faq/altimetry_faq.aspx