We know that there was a major fire in the cemetery in 1943, and another report just fire years later suggests the reason why more grave markers were lost or damaged.
The background is that this is a cemetery that appears to have suffered major neglect over the years. Whether this was aggravated by the location as much as the gradient and layout of the cemetery itself. How we know it today, is obviously a great improvement on how it has been cared for in past decades (thanks to Graeme P).
So, if you have been looking for evidence of a grave marker at Shortland Cemetery, don't rule out the possibility that it was destroyed by either the 1943 or 1948 fires. Below is the report of a fire that spread through the upper portion of Shortland Cemetery, Thames - in February 1948.
Source: The Thames Star Collection, The Treasury, Thames