The path of the plane may just have touched the radar's range at some point or some idea of the track may have been worked out.
[0] http://en.wikipedia.org/wiki/Jindalee_Operational_Radar_Netw...
Jindalee uses a directable beam to scan the area or interest. It takes hours to do a full sweep of its entire range (bear in mind that with over the horizon radars you have to do a 2D sweep, not the 1D sweep of a standard line of site radar). Generally speaking, the radar tends to be targeted on areas of specific interest, not just randomly sweeping (the sweep pattern depends on current threat profiles). Maybe if Malaysian authorities alerted Australia quickly enough they may have pointed the beam to the right area, but as I understand it the alert didn't start going out until much later. Disclaimer: ex RAAF radar engineer...
Some more info: http://www.abc.net.au/radionational/programs/breakfast/could...