I think the grant will be extended, but probably in a slightly lesser form.
At the moment it means that some buyers can purchase a property with no savings at all, they even get cash back when they settle. It's a dangerous path to go down (lending money to someone with no savings history).
I also think that the government is spending a lot of money and the current level is unsustainable.
Whilst the grant is having a positive effect on the housing market, I do worry that perhaps the government is throwing money at the problem a little too quickly and is not stopping to measure the results. Interest rates have dropped so far, so fast, that the real effect on the economy may not be know for some time yet, but there's already strong suggestions of further large rate cuts.
I also believe that too much government interference is potentially even more damaging. Recession is part of the economic cycle. Throwing money around to avoid it may mean that it's simply delayed and when it does come it's actually worse than it could have been.
As a case study, the US should have gone into recession in about 2001 - 2002. Instead they threw money at the housing market which resulted in todays unstable financial markets and massive oversupply. At the same time they stimulated the economy by starting a war in Iraq (wars are very profitable for certain economic sectors). The result is a country trillions of dollars in debt. At the end of the Clinton administration the US was in surplus.
The list of US government intervention which has directly effected todays financial crisis has numerous similar examples of short term problems with long term ramifications, as does Australian history.
I believe that government intervention is a useful tool, but one which should be used very carefully and delicately. At the moment it seems like they're applying economic intervention with a very heavy hand.