Actually we don't have 'freedom of speech' in Australia. We have freedom of political communication only. (An implied right from the Cth Constitution).
The issue of whether to allow her entry is interesting. On one hand I think, 'yes she should be denied a visa as the purpose for her visit is in opposition to our public policy of encouraging immunisation.' If she was coming as a tourist then I think she should be allowed a tourist visa. But coming for the purpose of giving seminar talks trying to persuade people to not vaccinate is really not something that's in our best interests as a country.
On the flip side I can see that its a dangerous slippery slope to deny people entry because the government doesn't agree with their views. That's censorship and it starts to get murky about what level of censorship is okay.
Overall - I fall on the 'no she shouldn't be given an entry visa' side of things. Her views are downright dangerous, and they are not political so don't fall within the right to political speech.