Why is it that the big lows seem to come North to the west of WA, then slink away as they travel east? Guessing its something to do ocean versus land

Not totally sure, but it has something to do with the way the systems tend to 'peak'. I think big lows move faster than high pressure systems, so they're always going to slide south at some stage.

Why they don't penetrate the east of WA is probably due to the lack of moisture over land. A bit of a disjointed explanation, but it's something. What I don't understand is why polar blasts always peak over Tasmania, Victoria, the Tasman or New Zealand, and rarely over the Nullarbor or Perth.

Perhaps it's more about the stationary highs that sit over the SE of Australia this time of year. The lows just get forced below these.

My met 101 is rusty & basic but at a guess there's more friction over land than water so it's easy for cold low p systems to spread north over water but much harder over higher friction & drier land (either south WA, SA or Vic).