Have been hearing this term a lot in the coverage of the hostage-taking today.
What is a “Sydneysider”?
Someone who lives in Sydney.
Much in the same way people who live in the US are sometimes called “Statesiders.”
Coming up with names for residents of awkwardly named cities is a unique exercise. I don’t know who establishes them and when, but the best they have come up with* for Australian cities are:
Sydney: Sydneysider
Melbourne: Melburnian
Brisbane: Brisbanite
Perth: Perthian
Adelaide: Adelaidean
All a bit crap, really.
*At least, I think these are right
I have a feeling people in Perth are called Perthsiders. Don’t hold me to that, though.
While “what is a Sydneysider” is a pretty simple question, “who is a Sydneysider” isn’t always so clear. Sydney’s not really a city, it’s a metropolitan area comprising almost forty cities/shires/municiplaities. And the 4 million population figure always cited includes places like the Blue Mountains. Generally, though, most people agree on the councils listed in the Wikipedia article, but it’s not uncommon to hear places like Campbelltown or the Shire referred to as “just outside Sydney” (even though when you drive into both of them from the south you’re greeted with “Welcome to Sydney” signs).
I wonder whether there are any phonetic trends where cities with certain sounds in them (or certain sounds toward the end) tend to get a specific demonym.
FWIW, in the US:
Boston: Bostonian
New York: New Yorker
Philadelphia: Philadelphian
Baltimore: Baltimorean
Washington, DC: Washingtonian (or, if you must, Warshingtonian)
Atlanta: Atlantan
Cleveland: Clevelander
Milwaukee: Milwaukeean
Los Angeles: Angeleno/Angelino (from Spanish)
San Francisco: San Franciscan
A bit of an outlier:
Newcastle: Novocastrian
(I was born a Sydneysider, but became a Novocastrian at the age of 9)
Or Manchester = Mancunian
Or Liverpool = Liverpudlian
Someone (who I think was being serious) once told me that the people of Liverpool were called Lilliputians.
Sydney = Cockroach
Melbourne = Mexican
Brisbane = Banana Bender
Adelaide = Crow Eater
Perth = Sand Groper
Brisbane = Cane Toad thank you very much!! 
OK point taken, Queenslanders are technically Banana Benders…
Thanks for the replies, all.
Where does the “-sider” suffix come from?
Resident of the State of Michigan: Michigander. (The females are Michigeese.)
Probably just alliteration.
Well, you have words like Tyneside, Humberside, seaside, hillside to refer to the vicinity of some geographical feature - either a specific feature like the river Tyne or the river Humber, or a generic feature like sea, hills, etc.
Then, from about the seventeenth century, you have the word “sider” to describe somebody who identifies with or aligns themselves with some cause or some person - in the English civil war someone might have been described as a sider with the King, or a sider with Parliament, or with Cromwell.
From these roots it’s a short step to, e.g. Tynesider, someone who comes from, lives in or otherwise identifies with Tyneside.
“Sydneysider” is one step further on, because in fact there is no area called Sydneyside, and no river, mountain or similar geographical feature named “Sydney”. But I think “Sydneysider” is formed on the model of “Tynesider”, etc, probably because the more regularly-formed alternative, “Sydneian” has just too many consecutive vowels for comfort.
Toronto -> Torontonian
You know, for the past 25 years I thought King’s Cross was where Syndey SAILORS meet! Consider me corrected.
Are people from the largest city in California called “Angelenos”? I know people who are from Taos NM are called Taoseños, but the name is strictly reserved for those who were born there.
Glasgow = Glaswegian
Galway = Galwegian
Cork = Corkonian
Dublin = Dubliner
Well, Hello Sailor…
A more contemporary view.