In the absence of further information I would suggest the plant is Ficus benjamina, or Benjamin Fig. For a test that it is in the fig family pull a green leaf from a stem and see if it has a milky sap. If not, I am wrong and can delete the suggestion.
Ficus benjamina grows outside in Sydney; there are some examples in the Sydney Botanical Garden. Generally they are discouraged in small gardens because the roots can be invasive and destructive to paths and buildings.
Normally they are grown from cuttings. It looks in this case as if someone took some cuttings (or sowed seeds close together, or even root pieces) and planted them close together with the intention of later separation. Towards the right to the back close to the fence it looks like someone cut back one of the vigorous shoots.
These are a type mangrove which can grow about anywhere abundant of water take notice of the leaves in theses examples below. Mangroves can grow on land a brush but not normally as a tree but I did find a dry land mangrove tree shown on the last picture. I cannot give you the exact species sorry.