Felicia's point is a good one - this is a discussion of the ISA scheme, right? The First Home Owner's scheme in Australia was, as far as I can tell, a grant that required a fairly simple application process to get - the ISA scheme here requires a) quite a bit more forward planning and b) a fairly modest rate of savings over a lengthy period of time.
If I understand the system correctly (please correct me if I'm wrong, Felicia, as I'm hoping to apply for one!) the ISA has to be started within a tax year in which you haven't paid into any other ISA, so if you already have savings in another ISA you have to time the switch carefully. Moreover, you can only put in up to a maximum of £200 per month. The government also contributes a percentage up to a maximum of £3000, but if you take the money out before you've "filled" the ISA then obviously you receive less.
This seems quite different from the Australian scheme in a number of ways. I think the complexity of setting it up means less people will apply, which on the one hand is exclusionary. On the other hand, the fact that it isn't a grant and is instead a reward for fairly low-level but consistent savings means that it is to some extent designed for people who are slowly trying to build up their deposit. It also incentivises saving consistently, as it commits you to that £200 a month to get the reward.
I don't think the uptake will be wide enough for it to have an impact on house prices (as it won't be taken up across the board as the Australian scheme presumably was). I think it will make a huge difference for individual young people looking to buy a first home in the £100,000-£150,000 range and may mean individuals or couples who take it up end up out of renting and on the property market a year or so earlier than they might have otherwise.
All that said, it obviously is a "sweetener" policy, but I don't think it will have quite the catastrophic effects people are predicting here...