An Air New Zealand flight to Houston has been diverted because of a medical emergency.

Flight NZ28 left Auckland at 5pm yesterday but was redirected to Honolulu "after a passenger became unwell during the flight and required medical attention", said Air New Zealand spokeswoman Kelly Kilgour.

She could not say what the medical emergency was for privacy reasons.

The aircraft refuelled and has since arrived in Houston about five and half hours later than scheduled.