An informal vote is one that has been incorrectly completed or not filled in at all. They are not counted towards any candidate.
According to Section 268 of the Commonwealth Electoral Act (1918), a vote is informal if:
The ballot paper is not marked at all.
The ballot paper does not have the official mark and has not been initialled by the polling official and the ballot paper is not authentic in the opinion of the Divisional Returning Officer (DRO).
The ballot paper has writing on it which identifies the voter.
In the case of an absent vote the ballot paper is not contained in the declaration envelope.
The voter has not marked a vote correctly for it to be considered acceptable.
A formal ballot paper must contain the number 1 and sequential numbers thereafter. Only one candidate may be left un-numbered because it will be assumed this is the voter’s final preference.
In the Senate, the voter must place a number 1 in a box above the line, OR number all the boxes below the line in sequential order.
The most common forms of informal votes are those where the ballot paper is blank, or where the voter has incorrectly numbered the candidates by only numbering one candidate or using non-sequential numbers or ticks and crosses.