If Finance Minister Grant Robertson expects this week's GDP data to deliver him a new bicycle for Christmas, he had better reckon on a box of crayons instead.

That's if bank economists' September quarter forecasts of around 0.4 per cent growth come to pass.

Growth at that level would take annual average GDP growth to 2.2 per cent, down from the June annual average of 2.7 per cent.

Three of the four big banks expect a 0.4 per cent increase in GDP, while the Bank of New Zealand is slightly more optimistic, with a prediction of 0.7 per cent for the quarter.

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Either way, it's a tepid growth rate, especially given the population increased by 2.1 per cent over the year.

"It's not too bad, but given the population growth that has happened underneath it, it's not what you would call a strong rate of growth," Bank of NZ senior economist Craig Ebert said.

The quarterly number is expected to show the effects of a weather-related decline in milk production and a natural decline in oil and gas production.

Furthermore, the fizz created in the tourism and accommodation sectors by the Lions rugby tour in June and early July will have dropped out of the September quarter GDP equation.

ASB said its "big picture" view of growth has been revised lower to an annual average of 2.2 per cent from its previous forecast of 2.5 per cent.

"At the start of 2017 our expectations for economic growth were perky," the bank said.
"We had expected low interest rates, still-strong population growth, construction demand and a lift in per-capita demand to boost activity. But GDP outcomes have underwhelmed," ASB said.

Economists said that if the Treasury's forecasts, released last week, prove overly optimistic, then the Government could end up borrowing more than it anticipated.
"We struggle to see how the Treasury's forecasts will be achieved over the three year time frame," BNZ's Ebert said.

The official forecasts from last week's fiscal update show annual average GDP growth slowing to 2.9 per cent in 2018 from the previous estimate of 3.2 per cent.

"Compared to the pre-election update, underlying momentum is a little weaker across the second half of 2017," the Treasury said.

"Economic growth appears to have slowed in the 2017 September quarter with wet weather hampering agricultural production, continued weakness in the housing market and slower growth in private consumption," it said.

However, growth is forecast to be 3.6 per cent in 2019 from the previously forecast 3.7 per cent and will lift to 3 per cent in the year to June 2020 versus a prior forecast of 2.8 per cent.

ANZ senior economist Phil Borkin said indicators for the current quarter looked a little stronger, which was encouraging.

"Nevertheless, while it should be emphasised that we retain a generally positive view on medium-term growth prospects, we do see the potential for growth over the next few quarters to remain somewhat mediocre and sub-trend," Borkin said.

"The economy is not only navigating headwinds from the softer housing market, but is also transitioning in terms of its growth drivers, in part due to capacity pressures," he said.

"Throw in some unease regarding the new political direction, and more recently the dry weather potentially hampering agricultural production, and we see reasonable prospects of a growth wobble," he said.

Economists said the wildcard in Thursday's release will be the possibility of big revisions to past data when Statistics NZ includes two years of annual benchmarking updates.