A lot! The average electorate has 100,000 enrolled voters but any election can see the slimmest margins deciding winners. Here are a few examples (Vote Australia does not imply that fraud occurred).
2016 - Herbert Qld - 37 votes -- http://results.aec.gov.au/20499/Website/HouseDivisionPage-20499-165.htm
2007 - Robertson, NSW - 184 votes -- http://results.aec.gov.au/13745/Website/HouseDivisionFirstPrefs-13745-146.htm
2007 - Flynn, Qld - 253 votes -- http://results.aec.gov.au/13745/Website/HouseDivisionFirstPrefs-13745-311.htm
2007 - Solomon, NT - 196 votes -- http://results.aec.gov.au/13745/Website/HouseDivisionFirstPrefs-13745-307.htm
2013 - McEwen, Vic - 313 votes -- http://results.aec.gov.au/17496/Website/HouseDivisionFirstPrefs-17496-226.htm
2013 - Fairfax, Qld - 53 votes -- http://results.aec.gov.au/17496/Website/HouseDivisionFirstPrefs-17496-160.htm