Visiting the cache site you will immediately be able to see the purpose of these objects. In the event of an invasion during WWII they would have been used to block the road up to the strategically important Taiaroa Head. This is pure speculation on the cache owner's part but I would imagine that any amphibious landing in the Dunedin area would have occurred on the beaches on the coast. However, gaining access to the calm waters of the harbour to safely unload supplies would have been a top priority for any invading force in the hours immediately following any landing. The gun emplacements at Taiaroa Head/Harington Point would have made sailing into the harbour a difficult prospect so one assumes that a priority for any invader would have been to move down the Otago Peninsula quickly and take the guns by attacking their rear. The road at GZ is the only one into the area so blocking it and forcing invaders to take a more difficult cross-country route would have been essential. You have to wonder how long these objects would have withstood attack but any time they bought for the defenders would have been crucial.
In 1942 the prospect of a Japanese invasion of New Zealand was high. After the surrender of the British at Singapore nothing stood between New Zealand and invasion. The bulk of NZ's own army was already fighting Rommel's Afrika Korps in the Western Desert of Libya and Egypt and it was clear that they would not be brought home to defend NZ. It was only the arrival of US forces that halted the Japanese advance. daywalk's Found log of 16/11/14 contains excellent information on the history of this object and is worth reading if you are interested
A cliff face right above GZ might cause signal bounce for some GPS receivers so the hide is straightforward and the hint explicit.
CAUTION: Parking is very limited at GZ, almost non-existent. A small car might be able to be parked at GZ but only after it had been driven up the hill, turned and brought back down. DO NOT attempt to veer onto the wrong side of the road to park at GZ as this is a narrow blind corner. Much, much safer if you park on the flat or drive up the hill and park on the wider shoulders then walk to the cache.