There's a list of the exemptions on the DoT's website:
Summary of Key Features The Curfew Sydney Airport has a curfew in place that applies to aircraft operations between 11.00 pm and 6.00 am (local time).
www.infrastructure.gov.au
I have been on a delayed flight that took advantage of this exemption and took off a few minutes after 11pm.
Looking at the MTOW of the Dash-8 Q400, I think that QantasLink probably could get the necessary noise certs and operate them after the curfew, but given their regional range it's probably not practical in demand terms. Who wants to fly down to Canberra at 11:30pm and arrive after midnight? There might be a handful of people if it was cheap enough, but certainly not enough to make it commercially viable.