The dry summer, particularly in the North Island, has been hurting dairy production.
Overall, Westpac economist Nathan Penny expected agricultural production to make a below-average contribution to growth over 2013.
Weather developments were being watched ''nervously'' and, at this stage, it appeared the dry weather would have a negative effect on GDP over 2013, he said.
He estimated dairy production in the South Island, which was faring better than the North Island, would be up about 10%, compared with last season.
While the dry conditions were not as severe as in the North Island, better access to irrigation, particularly in Canterbury, and dairy farm conversions were also contributing to strong production growth.
ASB economist Nick Tuffley said lower dairy production would have a direct impact on first-quarter GDP.
From an income perspective, reduced production would be particularly disappointing, as farmers were unable to benefit from the pick-up in dairy and meat prices over the past six months.
The impact of lower revenue would be further compounded by higher production costs, as dry weather would have boosted feed requirements.
That would have a lasting effect on the economy, as reduced profits would result in farmers scaling back investment next season, Mr Tuffley said.
The ANZ commodity price index increased 1% in February, representing the seventh consecutive monthly rise in the series.
Prices have increased 9% since the most recent low recorded in July last year, but remained 13% below the high measured for the index in April 2011.
Butter recorded the greatest individual lift, increasing 4% from the previous month. Wool and beef prices both dropped 2% in February, to four and three-month lows, respectively.
