This is simply wrong headed. The presumption of innocence only comes into play at trial, when it is assumed that the accused is innocent and the prosecution must prove otherwise, beyond reasonable doubt. It doesn't apply anywhere else down the line, at the investigation, apprehension or pre-trial stages.
If it did, it would be impossible for the police to arrest anyone for anything, no matter how glaringly obviously culpable they were: "Hmmm, so you're standing over your wife's dead body, covered in her blood and clutching a knife that matches the 23 stab wounds in her torso. But I have to presume you're innocent, so I'll let you go."
The police don't presume that people are innocent when they investigate them, or arrest them. They aren't presumed innocent when bail applications are made.
There is a parallel for this. During the debate over the repeal of Section 59, people were believed (because they were told this) that what was happening would affect their lives immensely. The truth was far simpler, of course. A defense that only applied in a court of law - like the presumption of innocence - was being removed.
The change placed no further requirement on police to investigate complaints or allegations - they had to do that already. Allegations were not more likely to result in prosecution, in fact, because of the 'public interest' provision now written into law, parents enjoy more protection than previously. ALl that changed was that if a person abused their child so severely that they were prosecuted and put to trial, they could no longer defend themselves by claiming they had been using reasonable force.
All of which goes to show that idiocy is not the sole preserve of the left or the right.
1 - A not untypical comment: "A man is presumed innocent until proven guilty." (http://indymedia.org.nz/mod/comments/display/82767/index.php)