We have a similar tree here called Blueberry ash
Elaeocarpus reticulatus
This is one of the few tree species that readily accepts a good topping and whacking!
Now left unchecked it grows to a tree alright, maybe the biggest I have seen is 40'
Problem is the stem diameter and roots get quite thick, even when hedged over time the stems get like 1' dia so I dont know where exactly you have them planted but some people here put them in narrow retaining walls against a fence ... sure disaster.
The other thing you need to consider is hedge trimming the tops is hard work especially when you go past shoulder height.
Working off ladders is dangerous and by OHS rules not allowed for hedge maintenance. That means scaffold or painters planks. Starting to see the troubles in having a species that wants to grow past it's "hedge height".
I have removed rows of these in unit complexes for that exact reason and suggest they replant with something that only grows 2m high. We have an Australian syzygium (lillypilly) called Aussie Boomer (PBR licenced plant) that is ideal and grows to 2m high but is bushy yet compact.
That's called smart gardening, think ahead, your hedge wants to grow to 60', in summer you'll be trimming it every month unless you fall of the ladder early.
Plant 6 Syzygium Aussie Boomer pBr