Westpac chief economist Brendan O'Donovan said GDP growth might be once again affected by measurement issues in the communications industry, he said.
Communications has been the fastest-growing industry for decades, averaging 2.2% growth per quarter in the 20 years to March 2008.
"From mid-2008, communications suddenly turned south and it's not all due to the recession. People are replacing traditional forms of economic activity, such as phone calls, with internet use."
Statistics NZ found it easy to monitor declining volumes in "old industries" - it knows precisely how many fewer minutes New Zealanders are spending on their telephones. But it finds it more difficult to capture the rise in internet-based economic activity.
Inevitably, some of the increased activity was missed, resulting in persistent understatement of GDP despite the statisticians' best efforts, Mr O'Donovan said.
"Frankly, there is no way of telling what measured communication activity will do this quarter."
ASB economist Jane Turner agreed.
Close watchers of quarterly GDP might recall her surprise at the rate at which communications activity had started to fall in the past three quarters.
"What really caught our attention were the three large consecutive quarterly declines, which coincidentally started around the time the new mobile competitor 2degrees started up. It also seems a bit strange that communications activity should suddenly fall so sharply given trend activity in communications only flat-lined during the recession."
On further investigation, there did appear to be some serious measurement issues with the category, she said.
The weight Statistics NZ was placing on each activity in the fast-changing sector was out of date, placing too much emphasis on activities there were falling out of favour and not enough emphasis on growing forms of communication.
Statistics NZ was partially rectifying that issue in the September quarter by recalculating weights based on revenue earned from each activity, she said.
The GDP figures are not expected to set any records for economic growth, with most commentators expecting a lacklustre set of figures for the three months ended September.
Westpac and the Reserve Bank are forecasting quarterly growth of 0.3% and ASB is forecasting 0.1%.
Domestic spending was slow in the quarter, despite previous optimism about a spend-up before the new 15% GST rate was introduced.
Mr O'Donovan said he expected the third quarter figures to display an element of post-drought recovery in agricultural production and downstream manufacturing - despite the September storms.
However, the Economic Survey of Manufacturing made it clear that, other than agriculture-related production, New Zealand's manufacturing industries were struggling.
Non-agricultural manufacturing volumes fell sharply for a second consecutive quarter, with the exchange rate playing an important role, he said.
Service sectors had "really sagged" over the past year and that trend was expected to have continued in the third quarter.
Government cutbacks were keeping a lid on growth in the health and administration sectors.
"Real estate and finance are dropping with the weak housing market and knock-on effects are curtailing growth in the business services and personal services industries.
"That said, we expect to see sparks of life in the retailing, wholesaling and transportation industries as sales of goods picked up sharply in the third quarter."
Mr O'Donovan warned against too much excitement because some of the pick-up in sales was pre-GST spending and there would be a corresponding let-down in the fourth quarter.
Explanation
• Gross domestic product (GDP) is a measure of the output produced in an economy during a period.
