I agree with Mrs Mac, I would give them the notice they require of your intention to take Mat leave and the date you intend to finish up (bear in mind if you currently work under a NSW award you are supposed to give them at least 10 weeks notice of your intention to take leave and 4 weeks notice prior to the actual date, so you might need to do that soon) and then decide on the bigger plan whilst you are on Mat Leave.

You have to do what is right and comfortable for you. Lots of families use childcare early on.

You have mentioned that your boss is great. Have you had a chat to him about your desire to return to work Part Time ?
If you are having trouble, or are not sure how to go about negotiating the best arrangements for you, maybe contact your Union or get in contact with the relevant Union and join up.

FYI - in NSW if an employee is employed under a NSW State Award, the Industrial Relations Act 1996, was amended a short time ago to reflect the push for more flexible, family friendly work environments.
In addition to the legislated upaid 52 weeks maternity leave available to everyone. If you work under a NSW State Award you may now request to increase that by a further continous 52 upaid weeks.
You now also have the right to request to return to work on a part time basis until the child reaches school age. The employer may only refuse a request for the latter two points on reasonable grounds related to the effect on their business or workplace.

Hope this info helps.... Best of luck.