Gross domestic product growth in the December quarter came in below forecasts at 0.3%.
The Reserve Bank had recently forecast quarterly growth of 0.6%.
Annual growth for the year ended December was 1.8%, compared with forecasts of 2.2%.
Foreign exchange markets reacted by sending the New Zealand dollar half a cent lower against most currencies.
Mr English said the economy had grown in 10 of the past 11 quarters since emerging from the recession, which started in New Zealand in early 2008.
The December quarter was challenging as the economy continued to rebalance. Households saved a bit more and were being careful with their spending. Also, conditions were tough around the world, particularly in Europe.
"Quarterly numbers do move around a bit - the September quarter was higher than expected while December was lower than expected. We're focused on the longer term by pressing on with the Government's programme to make the economy more competitive and productive."
The data released yesterday by Statistics New Zealand included signs the economy was benefiting from an increase in net exports and investments, which would help New Zealand over the long term.
"Compared to a lot of other countries, we're in pretty good shape," Mr English said.
Green Party co-leader Metiria Turei said the Government's focus on selling high-earning state-owned assets was a "sad sideshow" to much bigger structural problems going on in the real economy.
The percentage of small and medium enterprises involved in exporting had remained around 15% since National took office in 2008, according to numbers provided by the Ministry of Economic Development.
"One of the key measures of a more resilient and competitive economy was the number of firms exporting. Under the National Government, the number of companies exporting has stagnated," Ms Turei said.
Council of Trade Unions economist Bill Rosenberg said the economy was still going nowhere. The Government seemed content to have unemployment at more than 6% and wait for the reconstruction of Christchurch.
"Its cuts in spending are contributing to the stagnation. It's past time for a much more active approach."
While there was still good news in the agricultural export sector, falling prices might put that at risk. In the meantime, the domestic economy had contracted, he said.
"It is not clear that the so-called rebalancing of the economy towards exporting is anything more than increased agricultural production resulting from high commodity prices that appear to have peaked," Mr Rosenberg said.
Ms Turei said GDP statistics highlighted an increasing dependence on milk powder as a way for the country to earn its way in the world.
In the quarter, manufacturing fell by 2.5%.
"Agriculture is again providing the bulk of the impetus for growth in the New Zealand economy, while manufacturing continues to decline and with it the opportunity to export high value-added products."
Under National's economic leadership, the New Zealand economy was dangerously reliant on the fortunes of one commodity - milk powder - she said.
