McDonald's Greenlane has closed twice this year to be sprayed for flies, after customers complained of clumps of them on ceilings, chairs and tables.
Glen Miles, from Waterview, said the fourth birthday he attended with his children on Sunday was spoilt by the high numbers of flies.
"They were all over the ceiling, all over the doors, flying around, on coffee cups ... hundreds of them. My wife actually swallowed one of the flies, and that's when we realised they were everywhere."
Restaurant assistant manager Kiran Challa said regular spraying had not fixed the fly problem, and the building would be closed again on February 12 so that large parts of the ceiling could be replaced.
Auckland Council officials had investigated the McDonald's twice, and had been satisfied that it was safe to continue serving food.
Entomologists said higher temperatures meant flies developed faster, and produced more generations. The increased moisture during La Nina meant more rotting matter, which was an important food source and a key ingredient in breeding.
The flies are attracted to food scraps in homes and also to humans' sweaty hands which produce an acid they need. One inner-city dweller said fruit flies were camping out in his kitchen as if it was a holiday camp, but a good glug of bleach down the drainpipes and some 24-hour spray had sorted out their numbers.
La Nina's warmer conditions and northeasterly breezes were also responsible for the swarms of minuscule jellyfish on Auckland's east coast beaches. The tiny larvae thrived when the city's sheltered coastal waters reached around 20C.
Temperatures are expected to remain in the low to mid-20s in the North Island this week, with consecutive highs creating dry and settled conditions everywhere except the east coast until Friday.
A change is expected before the weekend with southwesterlies and rain predicted on Saturday and showers on Sunday.
