The new forecast makes assumptions about how much extra activity the KiwiBuild scheme will induce and about the impact of government policies intended to alleviate construction sector constraints, including migrant construction workers coming to New Zealand under a special KiwiBuild visa.
"There remains a high degree of uncertainty about the impact that these policies may have," Treasury said. "Growth in real residential investment may be weaker than forecast if capacity constraints are more binding than assumed."
That could also lead to KiwiBuild substituting for "other developments that would otherwise have taken place".
However, the Budget economic forecasts are bullish about the economic outlook over the next five years, driven by an expectation that immigration will fall by less than previously assumed, prices for New Zealand's exports will remain strong, and the global economy will continue to grow at a robust rate.
On top of that, the surge in government spending caused by the previously announced July 1 Families Package and other initiatives announced in the Budget are expected to underpin average annual growth of 2.9 percent over the next five years.
The forecasts expect real (inflation-adjusted) growth in gross domestic product in the year to June 2019 of 3.3 percent, followed by 3.4 percent in the year to June 2020, and then falling to 2.4 percent and 2.5 percent in the two following years.
These increases are one of the reasons for higher tax revenue forecasts over the next five years, which underpin a larger government spending programme while allowing it to declare Budget surpluses and meet its debt targets.
Inflation stays at or below 2 percent a year throughout the five-year forecast, lower than forecast average wage increases. Compared to the 1.6 percent wage rise in the year to June last year, the June 2018 annual increase in wages is forecast at 3.2 percent and averages 3.1 percent over the next five years.
The current account deficit remains stable at around 3 percent of GDP, unemployment drifts down close to 4 percent by 2020 and stays there, while participation rates in the workforce remain high by world standards at close to 71 percent over the whole five years.
While business investment takes a hit in the year ahead, with a forecast increase of just 1.4 percent, it recovers to a 5 percent increase in 2020, reflecting the Treasury's judgement that the current slump in business confidence will be short-lived.
However, it notes that "it is possible that uncertainty around any other reforms and their combined impact leads to more cautious behaviour" by investors.
However, much rests on the assumption that very current high levels of net immigration – 68,000 in the year to March – will fall to 25,000 by 2022. That is 10,000 higher than previous Treasury estimates, reflecting the agency's decision to reset its migration forecasting to mirror the reality of recent history.
However, it also publishes forecasts from an external agency, Sense Partners, whose forecast suggests net immigration could settle around 40,000 a year, based on combining five different forecasting models.
That would raise economic growth rates, increase employment, worsen housing shortages, probably spark higher house price inflation and is assumed to lead to higher interest rates.
A high-immigration scenario published in the BEFU raises the average growth rate over the next five years to 3.1 percent, produces a larger budget surplus than is officially forecast in the Budget, and would get the government to its Crown debt reduction target a year early.
Also published is a scenario in which global trade protectionism increases beyond levels already accounted for, knocking the growth rate over the next five years down to 2.7 percent a year, producing lower surpluses and causing the government to miss its 2022 target to reduce net Crown debt to 20 percent of GDP by 1.4 percentage points.
