Large waterbird breeding events are one-third as likely to occur in one of Australia's most important wetlands after floods were sharply curbed by expanded irrigation, researchers have found.

Major floods in the Narran Lakes on the Condamine and Balonne rivers in the northern Murray-Darling Basin are needed for species such as straw-necked ibis to breed.

Straw-necked ibis chick on Narran Lakes in the northern Murray-Darling Basin. Credit:Kate Brandis, UNSW

However, over the past five decades, water resource development has cut the frequency of these floods from one every 4.2 years to one in 11.4 years, according to a study published in Biological Conservation.

The restoration of environmental flows under the $13 billion Murray-Darling Basin Plan is predicted to lift the frequency of large flooding, the modelling shows.