If the particular situation
requires manoeuvring without a clearance
AIP ENR 1.1, 3.9 (General Rules, Operations in Controlled Airspace, Air Traffic Clearance and Instructions)
http://www.airservicesaustralia.com/.../1_1_1-116.pdf
If considered necessary, a pilot should request a different clearance from that issued. In an emergency, a pilot may act without a clearance and immediately advise ATC.