The Australian Magpie is not directly related to the Eurasian Magpie or Corvids and has a more pleasant voice.
It is related to Butcherbirds and the Currawong. There are several subspecies broadly split into the Black-backed and the White-backed.
The male of the nominal Black-backed, above (Sydney, Australia), has a dark black upper back.
The female of the nominal Black-backed subspecies, above, has a mottled grey upper back (she was with a Black-backed male in Sydney).
To complicate identification, White-backed males, Black-backed females and juveniles all have grey mottled backs. In addition, the several subspecies often interbreed.
The Australian Magpie has also been introduced into New Zealand where it is widespread (photos immediately above on North Island, New Zealand).
Seems to be a Black-backed male but the female(?) has a different back to the Black-backed in Sydney above.
Female White-backed Magpie (hypoleuca subspecies) gathering nesting material. Seems similar to the female with the Black-backed immediately above.
The Male White-backed above has white from the nape to the end of grey area on his back (and lighter eyes).