The thing that causes most votes to be declared invalid is if the member number is not written on the back of the envelope or it is written on the ballot paper.
There is, I agree, a terribly high level of informal votes and it doesn't reflect well on those who can't follow instructions particularly if there are only two candidates.
The NSW ballot paper and instructions were not wonderfully clear this time but even still, it is not hard to write 1, 2, 3, 4, 5 and your member number on the back of the envelope.