Reservoirs and dams in NSW are swelling, but experts say it may not be enough to boost wetland bird numbers.

A new study conducted by the University of NSW with four state governments has found that most bird species are struggling to find enough water to inspire large breeding events.

“It’s well below the long-term average, with even fewer birds than last year,” Professor Richard Kingsford, head of UNSW’s Centre for Ecosystem Science, has told reporters.

“All that rain that we had on the Great Dividing Range, some of it's runoff, but it’s been largely captured by the large dams and not got into the river systems.”

The Lachlan and Paroo rivers systems, which have relatively little irrigated agriculture, seem to be faring well.

“Those two river systems were really just pulsing; they were fabulous,” Professor Kingsford said.

But across the waterbird regions, total abundance, breeding, number of species, and wetland area are all showing “significant declines over time”, the report says.