Solved: I found out that the resolution of the volumetric light shaft is directly related to the resolution of the shadow map of that light. So in my scene setting the Shadow Resolution Scale to higher than 3 on the light resolves my issue. I tested this on a spotlights and directional lights and it works exactly the same on both.
On a directional light this could also be resolved by setting up an appropriate cascaded shadow map. i.e high rez shadows where you need high rez volumetric light shafts.
Hope this helps someone.