Is it legal under property law for a neighbour on top side of your land to pump out of creek running through both properties every day and use the water for irrigation of gardens?
Landholders with creek, river or lake frontage or who overlie an aquifer are allowed to pump the water for domestic use (including household gardens) and non-intensive stock watering. See section 52 of the Water Management Act 2000: http://www.austlii.edu.au/au/legis/nsw/consol_act/wma2000166/s52.html
It sounds as though your neighbour's activities are lawful.