Sounds like a timeous and good move by the WA government to go looking for new Perth airport sites. Multi-airport systems are an inevitable feature of major metropolitan regions, and Perth is heading towards becoming one of those. Securing the possibilities of future development by landbanking sites for new airports is good airport planning. Landbanking or securing land for the possible future development of a facility is a major way of implementing long-term development plans for a new airport at a reasonable cost. Buy the land well in advance when it is cheap. It is also a form of insurance which protects against the risk of needing a new airport site and not being to find one - particularly when the new site needs to be a couple of thousand hectares of vacant and reasonably flat land (as a guide, MEL is 2369 ha, and PER is 2105 ha).
The traffic threshold which seems to justify an effective multi-airport system is around 14 million originating pax per year for the metropolitan region. PER currently is 14 million pax per year, which is approximately 7 million originating pax. The current forecast is for 24 million by 2019 (which is 12 million originating pax and thus getting close to the threshold. Yes of course, forecasts go up and down and Perth in particular is proving very hard to forecast. But given that it will take at least 5 years to organize the landbanking (feasibility studies, engineering, environmental, public consultation etc), then now is about the right time to get it all started.