The NSW government is bracing for a slowdown in Sydney's property market, despite its predictions that the budget will be well in surplus over the next four years.

Treasurer Gladys Berejiklian​ will on Tuesday release the government's half-yearly budget review, which will show the government expects to record a surplus of almost $4 billion this financial year.

But the review will also urge caution about the continued strength of the property market.

About 12 per cent of government revenue comes from taxes charged when individuals and companies buy property, and almost 8.5 per cent of revenue comes from taxes charged on residential stamp duty.