I'm not an ornitholgoist so I had to look it up.
From Wikipedia
Swooping
Magpies tend not to be afraid of people, and they live in urban areas as often as in the bush, so magpies are a familiar sight to most Australians, and their melodic song is widely enjoyed. However, if magpies feel threatened while nesting (typically in August-September in southern Australia), even by an inadvertent intrusion into their territory, they will often swoop at the intruder in an attempt to drive them away. Magpies generally swoop from behind, and without warning, so attacks can be somewhat terrifying, particularly to children. For this reason, local authorities sometimes post warning signs during "swooping season", particularly in urban parks. Magpie attacks sometimes cause injuries, typically minor wounds to the scalp: however, this is uncommon. Cyclists can also be at risk, as they are unable to discourage the attack by turning to face the swooping bird: an attack may cause a cyclist to fall off or change course unexpectedly in traffic.
To avoid swooping attacks, the best course is to avoid the territory of nesting magpies during the relatively brief nesting season (magpies ignore people for most of the year). It is illegal to provoke magpies (by throwing stones at them or destroying nests) as it is likely to make them more aggressive and they are a protected native species.
If it is necessary to walk near the nest, some people prefer to wear protection. Magpies prefer to swoop at the back of the head. Therefore keeping the magpie in sight at all times can discourage the bird. Using a basic disguise to fool the magpie as to where a person is looking (like painting eyes on a hat, or wearing sunglasses on the back of the head) can also prove effective. Rarely, if a bird presents a serious nuisance the local authorities will arrange for that bird to be eliminated.