Low-lying Holocene coastal plains supporting freshwater wetlands are often close to or even below the level reached by the highest tides. The vulnerability of such freshwater systems to saltwater intrusion will increase should the sea level rise as a consequence of global warming. In areas with a large tidal range, one of the main processes inducing a reversion to saltwater influence is likely to be the rapid extension of tidal-creek systems. This is demonstrated with an example from the coastal plains of the Mary River in northern Australia, where tidal-creek systems have extended more than 30 km inland in 50 years, invading freshwater wetlands and destroying associated vegetation over an area of at least 17000 ha. Networks have grown at an exponential rate through a combination of headward extension along main channels and tributary development. Large tidal range, very small elevational differences over the plains, the presence of incompletely infilled paleochannels, and uncontrolled feral buffalo have been major factors contributing to the rapid rate of expansion.