The Department of Conservation is pleading with dog owners not to let their pets roam in nesting sites used by New Zealand's rarest bird - the tara iti or fairy tern.

The plea comes after a series of incidents apparently involving dogs brought to shore by boaties, then left to run freely in a fenced-off area of beach at Mangawhai.

Only about 40 adult tara iti survive in the wild, and of those just 10 are breeding females.

DoC tara iti supervisor Kallan Mehrtens said the latest incident occurred on Sunday evening.

"Someone landed on the beach and allowed their dog to roam unattended, and it entered the shell patch under our fences and ran around near four of the tara iti nests where we have brand-new eggs on the ground," he said.

"This kind of disturbance can cause our birds to abandon their nests. It seems to be getting more of an issue."

Mehrtens said rangers were on site during the day to keep an eye on the protected area, but the offending was taking place in the evening.

When rangers followed the dog tracks they led back to the water's edge and common landing sites for boats.

It seemed boaties, sea kayakers or even paddleboarders with dogs aboard were allowing their pets to run around on the beach and dunes, possibly to chase rabbits.

Mehrtens said only about 40 tara iti survived in the wild, so one careless dog owner could cause a huge setback to efforts by DoC, Auckland Zoo and local volunteers to help the species recover.

Tara iti made their nests in shelly parts of the beach along the east coast between Waipū and Pakiri, digging out shallow nests in the sand called "scrapes".

Mangawhai was a critical nesting area for the birds and also where DoC had a nursery preparing tara iti fledglings for the wild.