I asked on i-naturalist for others opinions who suggested this. Rottnest Island is protected from the mainland and therefore has only limited flora and fauna.
The internet states that there are only 2 types of snakes on Rottnest Island; they being the Rottnest Island Dugite (typically 1.5m long) and the Southern Blind Snake (Rena unguirostris), typically less than 30cm long and burrowing behaviour. The snake I saw was easily over 1m in length and is similar to a photo on the Rottnest Island website (subfile: learn/nature and wildlife).