Ugg boots : Australian sheepskin boots.
Uni : university
Unit : flat, apartment
Up oneself : have a high opinion of oneself - "he's really up himself"
Up somebody, get : to rebuke somebody - "the boss got up me for being late"
Useful as an ashtray on a motorbike / tits on a bull : unhelpful or incompetent person or thing - "he, she or it is about as useful as tits on a bull" etc. etc.
Ute : utility vehicle, pickup truck
Veggies : vegetables
Veg out : relax in front of the TV (like a vegetable)
Vinnie's : St. Vincent De Paul's (charity thrift stores and hostels)
Waggin' school : playing truant
Walkabout : a walk in the Outback by Aborigines that lasts for an indefinite amount of time
Walkabout, it's gone : it's lost, can't be found
Weekend warrior : army reservist
Whacker, whacka : Idiot; somebody who talks drivel; somebody with whom you have little patience; a dickhead
Whinge : complain
Wobbly : excitable behaviour ("I complained about the food and the waiter threw a wobbly")
Wog : flu or trivial illness
Wog : person of Mediterranean origin. A milder insult than the same word in the UK and perhaps elsewhere.
Woop Woop : invented name for any small unimportant town - "he lives in Woop Woop"
Wowser : straight-laced person, prude, puritan, spoilsport
Wuss : coward; nervous person or animal
XXXX : pronounced Four X, brand of beer made in Queensland
Yabber : talk (a lot)
Yabby : inland freshwater crayfish found in Australia
Yakka : work (noun)
Yewy : u-turn in traffic ("chuck a yewy at the next traffic lights")
Yobbo : an uncouth person
Zack : sixpence (5 cents) - "it isn't worth a zack", "he hasn't got a zack"