Spare a thought doomsayers to the statistic that 70% of Australian households own or are purchasing their home. For house prices to 'plunge/plummet/crash/whatever colourful adjective you want to use' you need supply to exceed demand (basically). Almost 3/4 of Australian households own or are purchasing their home - why would they sell? Even if they see house prices dropping why on Earth would they sell?
Once the system is leveraged then the movement happens almost entirely at the margins, with price discovery occurring generally when new money enters the market.
Thus your simplistic statement doesn't take into account macro factors that are hugely important and even misconstrues the idea of supply and demand:
1. Mortgage origination. For prices to continue up, the buyers must have hard cash equivalent to more than the current going price (highly unlikely) or the bank must be willing to lend it to them (much more likely). Considering the huge amounts of funding which the big 4 Aussie banks require from overseas to roll over debt, and originate new debt then the relationship between seemingly incongruous (they always seem incongruous in complex systems) factors becomes pretty clear. You used Greece as an example, it's a great one. How could a little tourist island in the Med have any impact on Australian house prices? Let me lay a hypothetical on you:
Greek bond crisis -> European bond market liquidity issues -> All marginal borrowers (our big 4 banks are a good example) shunted to the side and locked out of the market. Overnight and short term borrowing dries up or costs much much more and Australian mortgage origination rates decline massively in the same timeframe. Buying at the margins ceases, so there is a sudden huge mismatch between the number of buyers and the number of sellers.
2. Supply/demand: You state prices can only go down if everyone sells. This is a misrepresentation. In fact, if everyone who was going to buy has bought then there are no more buyers to continue pushing the price up. Even a normal level of supply will be able to drive prices down when there is no demand to fight it off.
So please, now that the FHBs are fully suckered in, explain to us all where the marginal demand is going to come from.
3. Nominal vs real (prices and income): if the price for productivity (i.e. wages) does not increase in line with the price of assets then the value of productivity is necessarily diminished. In the specific example of Australian home prices, this is definitely the case (it gets worse when you add health insurance, taxes, food and fuel, etc to the mix). The value of productivity has been diminished in regards to the purchase of a home and therefore demand is mathematically capped. Not to mention Australians have a history of being statistically less productive than almost all of their G20 counterparts with statistically higher inflation rates to boot.
Only those who are still being renumerated on an equivalent sliding scale to house prices can afford to keep on the right side of the line, anyone else falls to the wayside. The line used to move very slowly in an oscillating or cyclical pattern, now it moves very quickly in an inexorable forward direction. i.e. less and less people are on the right side of the line no matter how budgety and frugal they are and at the same time it requires more and more effort just to maintain the same position.
I can think of plenty more, but those are the 3 main ones that pop in my head. Demographics and our Governments incessant meddling in housing/building markets causing structural imbalance are others.