I'm onto it= I will investigate it/ deal with it immediately Example: Employer: Have you finished that report? Employee: I'm onto it and it will be on your desk in one hour.

I don't know about in Australia, however "I'm on it." and "I'm onto it." is completely different in the USA. I'm on it = I will start/begin or complete the task as soon as possible I'm on to it = Someone is trying to deceive me, but I know. I know the trick/deception, etc.