ACT rangers have identified a cunning male and female fox as top-order predators in the wetlands. The pair have wiped out hundreds of new baby turtles, due to be born in autumn.

During October, November and December the female turtles emerge from the water after rain, looking for open land away from the flood plain to nest. They urinate on the ground to help soften it before laying their eggs.

Ranger Michael Maconachie, who has captured the elusive foxes on sensor cameras, says the smell is a dead giveaway.

"Foxes get them [the eggs] that very same night. We're pretty certain foxes can detect the smell for two weeks," Mr Maconachie said.

Another ranger, John Freeman, said numerous turtles struck a netting fence near the water after they had emerged to lay their eggs, and consequently a lot of nests were built along the fence line. The two foxes then came along and raided them all.

Placing mesh over some of the nests, which each can contain up to a dozen or more eggs, did not stop the foxes from destroying the lot.

Excellent adaptors to the urban environment, the Fyshwick foxes are raiding nests despite intense scrutiny across the 201-hectare wetlands.

This was apparent on World Wetlands Day on Sunday which brought out in large numbers the bird-watching community, members of a managing trust for the reserve, rangers, Friends of Jerrabomberra Wetlands and scores of families who were taken on tours of bird hides.

Car parking, walking tracks, neatly mown grassed areas and a constant stream of bird watchers and photographers have not deterred the male and female foxes from making the wetlands their hunting ground. Adjoining the wetlands, construction crews are building the new Majura parkway.

Mr Maconachie said birdwatchers had seen the foxes take water birds, but rangers were reluctant to take out the foxes just yet, and were limited in their options. They cannot shoot them or lay 1080 poison bait because of the urban surroundings and domestic animals.

As well, the mature foxes dominated their territory, and removing them would open the way for new ones to enter and sort out a fresh hierarchy which could cause even more damage to wildlife.

"Knock down this pair and eight more foxes might start coming in to sort out new living arrangements," Mr Maconachie said. "Researchers in Melbourne and Perth are finding foxes take out 90 to 95 per cent of all nests, which is huge."

Wetlands management committee chairman Warren Nicholls said this raises all sorts of interesting questions.

"Questions like do we have a very old adult population of turtles, with no new generations coming on? We don't know if that is the case. Are there enough nests that allow some turtles to escape the foxes? We just don't know enough about it?"