A 1.8m tree suits a standard Australian ceiling of 2.4m, leaving room for a topper and air above it. Only go to 2.1m if your ceiling is 2.6m or higher.
Measure the ceiling, not the room
Tree heights are measured to the tip before a topper is added. A 2.1m tree with a 20cm star needs 2.3m of clear height, plus the height of the stand, which puts it against the plaster in most Australian homes.
Width matters more than people expect
A 1.8m tree is typically around 1m wide at the base. Measure the corner it is going into, including the skirting, before you order.
Fibre optic against unlit
Fibre optic means one lead to one power point and no string of lights to thread, which suits a room with limited power outlets. Unlit gives you control over colour temperature, and warm white reads better against a flocked tree than cool white does.