- The scientists involved used sensible precautions because they are pretty intelligent and have no interest in exposing themselves to dangerous pathogens.
- After the first sign of a dangerous outbreak the government gets involved and unlike movie governments they know what they're doing and soldiers will actually just sit in armored vehicles where the risk of zombie bites is minimal.
- It turns out brain tissue alone does not contain enough nutrients for a human-sized being to subsist for a significant amount of time and the outbreak naturally dies down after two days because the zombies run out of energy.
- In any case, Australia will be fine because zombies are not smart enough to pilot a plane or boat all the way there. If the outbreak starts there, the rest of the world is fine by the same logic.
Same thing how horror movies famously depend on the protagonists making the worst possible decisions ("let's split up!"). Zombie movies started off as societal critique about the dangers of consumerism. A realistic scenario does not make for an entertaining movie, and non-entertaining movies don't get funded.