Folks,
There are three kinds of "speed restrictions", regulatory, procedural and recommended. The last is somewhat rare these days, and not as important as the first two, but don't ignore one without careful consideration.
As has already been mentioned, the 250 below 10,000 in E and G in Australia is a regulatory limit, the only time you can exceed it is if there is a safety issue. "ATC" CANNOT give any kind of a release or waiver for a statutory speed restriction --- so don't bother asking. But please tell them when you are going to exercise your authority as PIC to exceed 250 below 10,000 --- for a provable safety reason.
The "speed limits" in D and C airspace in Australia are procedural, ATC can waive them when traffic permits. The PIC can decline to accept the ATC speed if it is a matter of operational safety, say a 290 kt minimum turbulence speed and there is turbulence.
Likewise, if it is a matter of operational safety (NOT commercial convenience/time saving) the PIC can exceed the 250 kt limit in E and G. Don't forget there is an evidential burden on the PIC to prove the breach was a safety issue;ie; guilty until proven innocent.
The US 250 below 10,000 (read the whole rule, FAR 91.117, very carefully if you are flying in US airspace) is regulatory, and applies to all classes of US airspace, which really means B, C and E, as there is very little G in the lower 48. Again, ATC CANNOT give a waiver or release from this restriction,(legally, ATC do not have a power to give an exemption from a regulation - to waive the law) but there is provision for the PIC to decide to exceed 250kt, if it is a matter of operational safety. See FAR 91.117(d).
250 below 10,000 is NOT the only statutory speed limit in US, read the rule very carefully. See the application of the 200kt limit, FAR 91.117 (b) & (c).
A number of countries have a mix of one or more of statutory, procedural and recommended speed, make certain you know what you are dealing with.
Tootle pip!!