Mystery fruit!

This fruit trees is in the grounds of Military Lines church in Tirunelvelli. I first saw it a couple or years ago but failed to take a picture.
I was delighted to see fruit on it again today. Does anyone know what it is?
A local man told me it was poisonous and that they just call it the unknown fruit. As it is in the ground of the old military church, it may have come from Europe originally.
The fruits are smooth skinned, hard to the touch, not citrus, ranging in size from four to eight inches diameter and green.
I like the picture but I'm not sure it gives any idea of the size or appearance of the fruit which look very odd and a bit like green balloons or footballs hanging from the branches.