Viscous effects on centreline shock reflection in an axisymmetric flow are studied numerically using Navier-Stokes and direct simulation Monte Carlo solvers. Computations at low Reynolds numbers have resulted in a configuration consisting of two shock waves, in contrast to the inviscid theory. On the other hand, computations at high Reynolds numbers have yielded a three-shock configuration in qualitative agreement with the inviscid theory prediction. This behaviour is explained by the presence of the so-called non-Rankine-Hugoniot zone, which accounts for the deviation of the shock structure from the inviscid paradigm. At Reynolds numbers on the verge of the transition from a two-shock to three-shock configuration, extremely high pressure that would be unattainable with the classical Rankine-Hugoniot relation for any shock configuration may occur. An analogy to the Guderley singularity in cylindrical shock implosion has been deduced for the shock behaviour from a mathematical viewpoint.