could anyone tell me the name of this water bird

Here is some info taken from BIBY:

The Mallard's natural range is across the Northern Hemisphere from the Holarctic region (North Pole to 30° - 45° latitude) to Hawaii, wintering in southern areas such as the sub-tropics of the Americas and south-east Asia.

The Mallard was introduced to Australia as early as 1862, with a rapid range expansion during the 1950's, and populations in suitable habitat areas are still increasing. In Sydney it was first introduced before 1900 and is now widespread in eastern New South Wales. In Victoria, the first introductions occurred in 1864, then in 1971 to 1972, and it is now widespread in the south. In Queensland, it has been found north to Maryborough, but no breeding has been recorded. In South Australia, it is found in the south-east. In Western Australia, it was introduced before 1912 and is found in and around Perth. It is uncommon but increasing in Tasmania and on King and Flinders Islands. It has also been introduced to New Zealand (from 1867), with several introductions from Britain, Australia and North America, mainly to augment populations for hunting purposes. It is also found on Macquarie Island, Lord Howe Island and Norfolk Island.