Auckland International Airport, New Zealand's busiest gateway, posted a 17 per cent gain in first-half profit and said its strong performance gave it the confidence to tighten the range of its full-year guidance for underlying earnings.

Profit rose to $165.9 million in the six months ended December 31, from $133m a year earlier, the Auckland-based company said in a statement.

Revenue climbed 6.9 per cent to $332m, which it said reflected a 2.7 per cent increase in aeronautical revenue "driven by passenger growth and increasing runway movements" partly offset by a decrease in international and regional aeronautical prices. Retail income grew 10.2 per cent.

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The airport company said full-year underlying profit would be in a range of $250m to $257m. That's a narrower range than the $248m-to-$257m estimate it gave at the time of its 2017 results.

The results come as the company gears up for a $1.8 billion infrastructure spending programme over the next five years, the biggest such investment in the airport's history to provide three more contact gates for international aircraft, a new domestic jet terminal, expanded border processing and public arrivals space, an upgraded international check-in area and investment in public transport, roading, and walking projects. It also plans to take steps in the next five years toward opening a second runway it currently expects to be required by 2028.

"The first half of the 2018 financial year also saw the company maintain its strong focus on upgrading its airport infrastructure and providing the best possible customer experience during a time of significant change," chairman Henry van der Heyden said.

"We continued to invest more than $1m every working day on our core airport infrastructure and there are now 53 aeronautical projects underway across the airport each in excess of $1m."

Auckland Airport's share of underlying profit from associates rose 47 per cent to $11.2m, with strong growth from Queenstown Airport, its share of the Novotel hotel and contribution from North Queensland Airports.

The company will pay a fully imputed interim dividend of 10.75 cents a share on April 5 with a record date of at the close of business on March 20. The payment is up 7.5 per cent from a year earlier.

Auckland Airport shares last traded at $6.285 and have declined 3 per cent this year.