Mr Barr would not release the June quarter financial statements for 2016-17, saying they would be released on Tuesday. But late on Monday the results were posted on Treasury's website.

They show a deficit of just $11 million for the year, a big improvement on the $74 million deficit forecast at budget time in June and a bigger improvement still on the deficit expected when the 2017-18 budget was released, of $182 million.

The last balanced budget was in 2010-11. While this year's results are interim and yet to be audited, the big improvement on forecasts will surprise economists who cast doubt on whether a surplus was achievable.

The improved result is delivered partly on the back of higher income from stamp duty - the tax on house sales, presumably driven by higher house prices.

Stamp duty is up $49 million, to $316 million, continuing its year on year increase, despite the government promising to phase out the tax as part of its tax reform shift from stamp duty to rates, which began in 2012. It has been cutting stamp duty slowly, but increasing house prices look to be more than compensating.

Rates revenue for the year is up $5 million on the budgeted figure, to $452 million for the year.

The financial statements say the improvement reflects higher than expected tax revenue and higher than anticipated returns from the Land Development Agency. It also reflects lower than forecast expenditure because of the timing of projects, and the capitalisation of repairs and maintenance.

Land sales of undeveloped blocks have come in at $132 million, $38 million higher than budgeted. Health payments from NSW for interstate patients are $29 million higher than expected.

In June, economists Pegasus told the ACT parliament's estimates committee that the return to surplus rested on a "problematic" assumption that higher Commonwealth spending would drive a strong ACT economy. The 2017-18 budget relied on an extra $126 million in Commonwealth grants from July and an extra $75 million in ACT taxes, Pegasus said, also highlighting savings that had not been explained or detailed.

A former senior Treasury official, Khalid Ahmed, pointed last year to a major contraction in spending that was built into the budget, with spending forecast to grow 3.7 per cent over the coming years, well down on the trend rate of 5.4 per cent over the past seven years.

In June, Mr Barr forecast the 2017-18 budget deficit at $83 million. It is unclear whether that is still the expectation, in light of the 2016-17 result.

A return to surplus has been on the horizon of successive budgets, but has been pushed back year by year.