The commodity boom lifted exports to a record $4.5 billion last month.

Exports exceeded imports by $464 million, outdoing market expectations of a $350 million surplus, even with the lumpy import of $270 million worth of aircraft.

But on an annual basis the trade surplus continued the declining trend evident since late last year.

For the year ended March the trade surplus was $631 million, having peaked at $1.4 billion in November last year. That is despite export commodity prices hitting new highs each month.

New Zealand has run trade deficits for most of the past 15 years.

Exports in March were 11 per cent higher than in March last year. ANZ's export commodity price index rose 24 per cent over the same period in New Zealand dollar terms.

Dairy product exports were up 26 per cent, forest products up 24 per cent and meat up 10 per cent.

Oil exports were 25 per cent lower than a year earlier - the volume had more than halved but prices were 30 per cent higher.

Imports, at $4.1 billion, were 17 per cent higher than in March last year. Excluding large aircraft, imports would also have been 11 per cent up on a year earlier.

Imports of machinery were 12 per cent higher than in March last year, not counting transport equipment which was up 380 per cent (including the aircraft). But imports of consumer goods were just 1 per cent higher than a year earlier and imports of cars 3 per cent lower.

"Export receipts remain strong, but surging fuel prices and the import of large aircraft have offset the gain from the strong commodity export performance," ASB economist Jane Turner said.