Since the mid-1990s, Melbourne has maintained significant population and employment growth. There has been substantial international investment in the city's industries and property market. Major inner-city urban renewal has occurred in areas such as Southbank, Port Melbourne, Melbourne Docklands and more recently, South Wharf. According to the Australian Bureau of Statistics, Melbourne sustained the highest population increase and economic growth rate of any Australian capital city in the three years ended June 2004. These factors have led to population growth and further suburban expansion through the 2000s.

Answer this question, if possible (if impossible, reply "unanswerable"): How many years did Melbourne sustain the highest population increase and economic growth?
three