Hey, Rowan - you forgot:
"Two snags (sausages) short of a barbie (barbecue)";
"Two sangers (sandwiches) short of a picnic"
"two stubbies short of a six-pack".

All mean someone isn't as clever as they could be (A 'stubbie' by the way is a small [about 375ml] bottle of beer)

"Thick as a brick" or "Thick as two short planks" - again - not clever

And one of my favourites - as it is all over the world one can buy beer here in six-packs of cans which are all held together with a plastic thing (apparently seals, dolphins and 'greenies' hate them) - I call it a plastic 'thing' because it's part of the colloquy:

We often refer to someone as "He's got the six-pack - he just lacks the plastic thingy to hold them all together"

Muttley