As long as your qualifying course was approved by the WFOT, it doesn't matter if you've got a BSc or an MSc. A post-registration MSc is not required and does not give you additional points. You can check all approved programmes here:
https://www.wfot.org/programmes/education/wfot-approved-education-programmes
The OT Council of Australia assesses skills for immigration purposes and they have a step by step guide on their website.