Exports exceeded imports by $600 million last month.

While the surplus was smaller than the $1 billion economists had expected, it represents $1.13 of exports for every $1 of imports, which is twice the average May surplus over the last five years by that measure.

In seasonally adjusted terms exports were down 8.1 per cent on April, driven by an 18 per cent drop in dairy export volumes despite good late-season production conditions. But that drop reverses a similar jump in export receipts between March and April.

Compared with May last year exports were 10 per cent higher at $4.6 billion. That included $95 million of pleasure boats to the Marshall Islands - not normally prominent in the list of export destinations.

The main agricultural commodities posted gains on a year earlier - 8 per cent for dairy products, 12 per cent for meat, 22 per cent for forest products and 15 per cent for fruit - notwithstanding a 2.5 per cent rise in the trade-weighted exchange rate over the year.

ASB economist Jane Turner said forestry export volumes had grown very strongly in recent months, largely on the back of demand from China.

"Recent values have not been quite as strong as volumes and suggest forestry prices may be slipping. Anecdotes suggest Asian demand is starting to slow, particularly as monetary authorities in China are looking to engineer a soft landing," she said.

Imports at $4 billion were 17 per cent higher than in May last year.

The imports side was swollen by the one-off import of $214 million of aircraft parts and an increase of $118 million in crude oil imports.

Oil is imported in large, irregular shipments which can throw monthly numbers around. In this case the volume imported was 100,000 tonnes or 26 per cent higher than in May last year, while the price was only 5 per cent higher.

Over the whole year to May exports of oil and oil products offset 30 per cent of imports in that category.

Imports of plant and machinery were up 21 per cent and industrial supplies (excluding oil) up 30 per cent - positive signs for business investment and industrial activity, said Bank of New Zealand economist Craig Ebert.

Imports of consumer goods were 5 per cent higher than in May last year and car imports 18 per cent lower.

"This, along with another decent enough result for exports in May, is good news regarding ongoing rebalancing of the economy," Ebert said.

For the year ended May exports exceeded imports by $1.1 billion, equivalent to 2 per cent of exports.