It's 750 hours total from when you commenced instructing ie the day your Gr.3 was issued.
The 500 hrs must be items in the Day VFR syllabus ie instruction for the purpose of training someone anything else doesn't count eg agricultural rating, instrument rating, formation & aerobatics endorsements etc etc
You must have held a Gr.2 for at least 12 months prior to sitting the flight test. You will also need to pass the Gr.1 theory exam and have a CFI recommendation.
The Gr.1 automatically gives you IR instructing privileges so you may have to give a brief or lesson about it.
Also you gain Instructor training & junior instructor supervisory privileges so make sure you know what they can & can't do.
Ditto the potential to be a CFI so make sure you're familiar with that section of the CAO ie 80.xx (or is it 82.xx?)