I need to gar-gull.

I got a shock when I first came to Australia. The seagulls all sounded like they had a cold. Unfortunately my Field Guide to Australian Birds by Pizzey and Knight doesn't describe the call of the gull I was used to hearing in England so you'll have to make do with my attempt to put it into words: it is a high pitched "co-loy co-loy co-loy co-loy!". The book does have a fair bit about the Silver Gull which is what I reckon this photo is of.

   "Voice: "short, harsh, gutteral 'korr' or 'keow'; longer, harsh descending 'karrr-karrr-karrr'."  

 ....in other words like they have something stuck in their throats. Anyway, reading into it a bit, I see that there are several different types of gull in Australia with different voice descriptions. At the risk of going into it far too much:

Pacific Gull: shouted 'ow! ow!; muffled 'auk, auk' gruff chuckles; whining stutters
Kelp Gull: yelping or laughing 'yo-yo-yo-yo' (I think that might be the one I know from England)
Black Headed Gull : high-pitched harsh screams in quick succession: 'kreeooo' or 'kraaahr'
Black Tailed Gull: plaintive mewing
Franklin's Gull: shrill 'kuk-kuk-kuk'; also nasal 'kear, kear'
Laughing Gull: strident laugh or variety of low chuckles
Sabine's Gull: harsh grating and tern-like 'k-year'
Lesser Black-Backed Gull: similar to Kelp Gull.

There now. More than you will ever need to know about the various calls of Australian Gulls.