I've discovered during my relatively recent introduction to the sport (chess is a sport too, right?) that there's not a lot of information on the interwebs about Sydney karaoke establishments. Here's what I've experienced to date:
Karaoke World - 185 Elizabeth Street (near Sheraton on the Park) - (02) 9267 5011
My first karaoke experience took place in these hallowed halls. They actually do have a fairly comprehensive website, which is partly why I used them to book a room for my 21st (all the way back in 2007!) They don't seem to have upped their prices since then, at least on the website. This place provides books with codes for songs in them, if you prefer picking your songs that way, which I can't remember having seen anywhere else.
Big Echo - 104 Bathurst Street (next to Oporto, near corner with Pitt Street) - (02) 9283 2666
A favourite at my last workplace due to the $10 6-9pm happy hour (hours?) promotion, including free Asian drink (lychee water, anyone?). Oporto being next door is handy for those late-night munchies.
Viva - 210 Clarence Street (near QVB) - (02) 9283 7733
Another favourite at my last workplace as it ran the same $10 for 3 hours with drink promotion. Also was relatively lax with BYO until recently. Probably has the best selection of English songs along with Karaoke World of the Sydney karaoke places that I've been to. There's a Japanese grocery store (along with the excellent 100 yen shop!) across the road for snacks. Has also been recently refurbished and ran a Ladies' Night promotion where girls got in free on certain nights, but that's unfortunately ended now. Still wondering where you've heard of it before? A man died after a brawl there in December 2009.
The original Greenbox on Pitt Street is now a Japanese bbq restaurant called Shinara. Greenbox Mark II, named Greenbox
Phoenix - 90 Hay Street (in the Chinatown pedestrian mall) - (02) 9211 1776
The only place I can remember that involves going up more than one flight of stairs for a k session. While that was fairly novel, we won't be returning here anytime soon. I've been told they've already cleaned it up a bit from the good old "hire a room complete with companions that will cater to your every need" days, but it was still pretty seedy last time we went. Plus, of course, someone got stabbed there in 2008.
CEO Karaoke - Basement (where else?), 1 Dixon Street - (02) 9283 7883
It has a website! A bit more upmarket than the others, being modelled on the chain of CEO karaokes in HK. Cleanliness on par with Greenbox Plus or thereabouts.
Mizuya - 614 George Street (near the Metro Theatre) - (02) 9266 0866
A relatively recent addition to the Sydney karaoke scene. A package for 10 people set us back $488, including all food and beverages up to that amount and 3 hours' room hire, which is alright value taking into account the range of food and drinks (if you eat/ drink more than the basic $488 amount, you just pay for the extra). The first place I've seen in Sydney with touch-screen song picking systems. Rooms were rectangular though which makes it harder to mingle.
All of the places seem to have cracked down on BYO, which definitely wasn't the case back when I first started going. But of course, going to karaoke isn't just incidental to the primary purpose of drinking, right..
Other places to try which I haven't been to myself: Redbox, Cashbox (? chapflap's not sure if this is still around), K Square... actually I lie as I have been to K Square (Capitol Square building), just that I don't remember so much about it. Same with Echo Point that I've been to once only (apparently it's haunted).
Outside of the CBD there's Ding Dong Dang (Surry Hills), Swing Box (Neutral Bay - we only found out about this place on a late night visit to Maisy's... there was just a door which looked kind of freaky), Juju's (Kings Cross - a Japanese restaurant which has karaoke late at night... I've been for the food which was good)... let me know if you konw of any others!