It's also worth noting that if you look for items within the last year, the Google web search pulls up 78 iterms for Pinjarra Massacre and 57 for Battle of Pinhjarra.  Refine that item to Australian items (as this is an article which falls under the Australian English tag) and Pinjarra Massacre gets 60 hits, against 38 for Battle of Pinjarra.  It looks very much that usage has changed, especially in Australia.