Upvote:1
Wotif is the dominant player in the last minute specials in Australia. They offer a pretty big range as well as reasonable discounts on or close to the day. They used to specialize in bookings under a month in advance but have slowly increased their window out to three months now.
Other than that, most large booking sites run an Australian branch or offer Australian hotels - hotels.com, lastminute.com.au, hotelclub.com.au.
Upvote:4
Well presuming you're ok with hostels, there are plenty of hostels available in Sydney - although you may have to call them when you get there to confirm the availability of beds for a last minute booking.
In addition, like in the US, Lastminute exists for Australia too.
Finally the one I'd recommend is booking.com - it can do same-day bookings and has the ability to search both hostels AND hotels. I just checked for tonight and indeed there are both hostels and hotels available in Sydney for booking.