Enter Oz on tourist visa, and immediately apply for spousal?

I'm an Aussie, and my wife is Japanese. We've been living in the UK since getting married last year, and she's having a baby in April. We want to move to Australia before then, and ensure my wife is covered by medicare before having the baby.

I think out best plan is to move to Australia soon (February), with my wife entering on her tourist visa, and then apply onshore for the spousal visa. According to the medicare/visa websites, immediately after application she should be eligible to apply for Medicare.

My question is - Is it 100% OK for her to enter on a tourist visa with a view to then applying for another one ? Or, at the border will they ask about the pregnancy, and whether we're planning on changing visas, and possibly deny her entry?

I think it's too late at this point to apply for the spousal visa while we're outside Australia, and we've got a time limit here....