The Australian Rank structure mirrors that of the British Army. Comparisons to Warrant Officer Ranks with those in the US Army are as outlined below:

US First Sergeant = UK/Aus Warrant Officer Class 2 (WO2) Company Sergeant Major (CSM) Squadron Sergeant major (SSM)
US Master Sergeant = UK/Aus WO2 (All other appointments)
US Sergeant Major = UK/Aus Warrant Officer Class One (WO1) All other appointments less Regimental Sergeant major (RSM)
US Command Sergeant Major = UK/Aus WO1 (RSM)
US Sergeant Major of the Army = UK/Aus WO1 (RSM)

In the British Army, a Warrant Officer is a Senior NCO rank not comparable to the various grades of Warrant Officer in the US, A Warrant Officer holds the Queens Warrant (Not Commision) and is usually referred to by their appointment ie.
WO1 (RSM) = Regimental Sergeant Major,
WO2 (RQMS) = Regimental Quarter Master Sergeant,
WO2 (CSM)/WO2 (SSM)/ WO2 (BSM)/ WO2 (ASM) = Company Sergeant Major/Squadron Sergeant Major/Battery Sergeant Major/Artificer Sergeant Major.

There are also inferred levels of authority within a single rank ie. a RQMS has a bit more clout within a unit than a CSM/SSM etc. because he holds a Regimental appointment.

The inferrence I gather is WO's in the British Army are a culmination of all aspects of their military track whereas US Warrant Officers are single track specialists and hold (in some cases) equivilent status and privilage to Commissioned Officers.

The most senior appointment for a WO in the British Army is Royal Logistics Corps Conductor, of which there are 15 or so (give or take), including 1 from the Territorial Army.

The next senior WO in the British Army is the Academy Sergeant Major (AcSM) who is the Senior Non-Commissioned Officer Instructor at the Royal Military Academy Sandhurst and is always from the Guards Division.

The senior Warrant Rank in the Australian Army is the Regimental Sergeant Major of the Army (RSM-A) and was introduced in 1991.

I once met an RSM-A in Frommelles in France in 1998 at the unveiling of the 'Cobbers' memorial (sculpted by Peter Corlett) at VC corner, he was a nice Bloke! in a yes sir, no sir, kinda way...