There are several things that render a ballot paper informal. Rules of formality differ between Federal and State Electoral Bodies.  There is a mandatory preferential voting system in the federal election for the House of Representatives, by which a ballot will be considered informal if:
These ballots do not count towards any candidate, and are counted separately. For analytical purposes, the AEC sorts and categorises informal ballot papers and examines them. The amount of informality is potentially influenced by a large number of factors. In the Australian context these factors include differences in the voting systems between some the States and the Commonwealth, differences in the voting arrangements between the House of Representatives and the Senate, the number of candidates, compulsory voting and sociological factors.
For additional resources on informality, the AEC has published two papers: "Informal Survey - House of Representative Elections 2001" , which analyses reasons for the current informality levels in Australia, and "Analysis of the Increase in Informality during the House of Representatives 2004 Election" which identifies factors that may explain the rise in informality from 4.80 to 5.18 percent between 2001 and 2004.  The analysis and research conducted in the 2001 survey of informality identified a number of factors, which may contribute to informality levels. The analysis revealed that electors from non-English speaking backgrounds were more likely to vote informally.