Despite earlier assertions that most mammals are colour blind, colour vision has in recent years been demonstrated in a variety of eutherian mammals from a wide range of different orders. This paper presents the first behavioural evidence from colour discrimination experiments, that an Australian marsupial, the tammar wallaby (Macropus eugenii), has dichromatic colour vision. In addition, the experiments show that the wallabies readily learn the relationship between the presented colours rather than the absolute hues. This provides a sensitive method to measure the location of the neutral-point, which is the wavelength of monochromatic light that is indistinguishable from white. This point is a diagnostic feature for dichromats. The spectral sensitivity of the wallabies' middle-wavelength-sensitive photoreceptor is known (peak: 539 nm) and the behavioural results imply that the sensitivity of the short-wavelength-sensitive receptor must be near 420 nm. These spectral sensitivities are similar to those found in eutherian mammals, supporting the view that the earliest mammals had dichromatic colour vision.