Higher than expected prices for the state's power assets and a still strong property market have delivered the Baird government a bonus $863 million in stamp duty, over and above forecasts made in the June Budget for 2015-16.

But the government is sounding a warning that the windfall stamp duties from the buoyant residential property market are coming to an end.

The budget half-yearly review, which will be released on Thursday, will reveal that the state government is expecting a massive bonanza from stamp duty.

Some $438 million will come from the transfer duties on the sale of Transgrid, which was sold earlier this year for $10.25 billion to a consortium, led by investment fund, Hastings and backed by a Canadian pension fund, and a Middle eastern investment fund.