The Great Barrier Reef (GBR), northern Australia, is the largest coral reef system in the world and provides habitat for highly diverse tropical marine ecosystems. Mixing in the coastal waters of the GBR is an important parameter influencing the health of these ecosystems. We have used the distribution of the four naturally occurring radium isotopes to determine the rate of mixing of nearshore waters of the central part of the GBR lagoon with water from the Coral Sea. The observed radium distribution is modeled using a one-dimensional diffusion model. The model improves on previous radium offshore mixing models by incorporating the benthic flux of radium diffusing across the sediment-water interface and offshore changes in water column depth. We find that the inner lagoon diffusivity (<20 km offshore) is best estimated using the short-lived isotopes 224Ra and 223Ra. The concordance of K x estimated using the two different isotopes and the apparent consistency between measured riverine inflows to the lagoon and inflows inferred from the modeled salinity distribution provide confidence in the results. The mean value of K x for the inner lagoon region of the southern central zone between latitudes 15.8¿S and 19.0¿S (265 ¿ 36 m2 s-1) is more than twice that in the northern central zone (14.3¿S to 15.8¿S). This difference likely reflects the different reef matrix density in the two zones. The distribution of the longer-lived isotope 228Ra indicates more rapid mixing in the middle and outer lagoon. These results indicate that central GBR water within 20 km of coast is flushed with outer lagoon water on a timescale of 18--45 days, with the flushing time increasing northward.