No travel medical insurance will cover prenatal care and childbirth.
Which province are you guys planning on moving to? From my own experience, Alberta Health Care Insurance Plan covers spouses and children on visitor visas if another spouse is a Canadian citizen or PR and Alberta resident.
After we submitted our PR application INLAND, we applied for visitor status extension and it was extended for a year "automatically" because there was a PR application in process. Is the same true for OUTLAND applications??
Not only Australians are rarely called for a personal interview at the embassy, but especially pregnant wives or spouses with children... "Yes, it's a marriage of convenience, that's why we're starting a family together"... (sarcasm)
My only question is, would there be any risk of troubles with immigration at the airport in Canada upon arrival? When I flew in with my pregnant wife on TRV, we were very close to being sent back due to high suspicion of her staying in Canada permanently... But they did let us in eventually, having grilled us quite enough... I don't know was it a rare precedent or not? Maybe nothing to worry about in your situation...