Feb 2011 update. Brian Conder has e-mailed in the following information “The structure shown in the photographs is indeed an air raid shelter. Can't remember how many there were in total, but there was definitely another one located in what is now Greycote, about 200yds south of Central Avenue on the left side of the road.
These were of reinforced concrete with a square opening at the opposite end to the entrance which presumably served as an 'escape hatch'. It was about 30" square inside, with an iron rung ladder from the floor to the top of the opening. Although the photograph shows the shelter in 'bare' condition, I'm pretty sure they were originally covered in soil and grassed over. Many school holiday hours were spent playing in and running along the top of the shelters which would have been difficult to climb without the aid of a grassy bank, although the top was fairly devoid of soil due to erosion caused by weather and wear and tear by children." Thank you once again Brian!