We have calculated binding and transition energies of the ground and some excited states of a shallow donor impurity in a disc-shaped GaAs quantum dot (QD), under the action of a magnetic field applied in the axial direction. The binding energies were obtained using the effective-mass approximation within a variational scheme, assuming an infinite confinement potential at all surfaces. Our results were obtained for several dot radii, the impurity position along the z-direction, and as a function of the applied magnetic field. We found that some excited states are not bounded for some values of the radius of the dot and of the applied magnetic field. We have shown how the applied magnetic field split the degeneracy of some excited states. Also, we have compared our results with those found in GaAs-(Ga, Al) As quantum wells (QWs) and quantum-well wires (QWWs).