Although most snake species are tropical, very little is known about their biology. We monitored reproductive histories of individually marked natricine colubrid snakes (keelbacks, Tropidonophis mairii) for 18-months in a tropical Australian floodplain. Our data provide the first unambiguous records (for any snake species) of individual females in a natural population producing multiple clutches of eggs during a single breeding season. The snakes bred throughout drier months of the year. The mean size of reproducing females decreased over the season, whereas relative clutch mass increased significantly over the same time period. Larger females produced eggs that were not only larger, but also shorter and wider, than those of smaller females. Despite producing two clutches in short succession, female keelbacks grew over the intervening period. Thus, they switch from ‘capital breeding’ (building up stores of energy during the wet season to use for reproduction in the dry season) to ‘income breeding’ (relying on recently ingested energy to support both growth and reproduction during the dry season). Two females marked at hatching were recaptured as breeding adults <12 months later, confirming that maturation is attained more rapidly in this species than in temperate-zone natricines.