Tree ferns don't flower and fruit as such, they produce spores like ordinary ferns do for reproductive purposes. The 'trunk' is simply a mass of roots, and cannot produce fruit, so whatever that is, either someone's put it there for fun, or its got a problem.

could it be an apple from a neighbouring tree. but as bamboo says they don't produce fruits, just spores.

Thanks bamboo and seaburngirl for your answers. I thought as much but it just seems strange as cannot see any similar fruit around the garden.

Have you tried picking it off? I would, and cut it in half to see what it is.

Nope, Snoop, can't be - the growing point is always at the top of the trunk, never lower down.