I also haven't been able to find anything in AIP, CAOs, CARs etc with specific reference to AMSL or AGL. The editors of AIP may be allowing assumption to be the mother of.....
From the Bureau of Met website:
http://www.bom.gov.au/general/reg/av...lanmetar.shtml
METAR/SPECI Format Explanation
Cloud height is given as a three-figure group in hundreds of feet above the aerodrome elevation, e.g. cloud at 700 feet is shown as 007.
http://www.bom.gov.au/general/reg/av...lanarfor.shtml
AREA FORECAST Format Chart Explanation
Description of cloud follows after the word 'CLOUD',
Using the following criteria.
- any cumulonimbus;
- any cloud at or below 5000 feet above the highest terrain in the area concerned by the forecast;
- any layer of more than 4/8 base at or below 21 000 feet above MSL, and
- any cloud associated with moderate or severe icing, moderate or severe turbulence, or precipitation mentioned in the forecast.
- The term 'LAYERS' is used if ST and/or SC are involved.
- When CU and SC are forecast and expected to be SCT or BKN, they can be combined as CU/SC.
- If subdivisions are used and one or more subdivisions have no cloud associated with it, the format is, A: NIL CLOUD.
- Cloud amount is given as 'SCT' (1-4 oktas), 'BKN' (5-7 oktas) or 'OVC' (8 oktas), except for cumulonimbus.
- Cumulonimbus cloud is mentioned first, then cloud with the lowest base, the next higher base second and then following cloud in ascending cloud base height.
- Cumulonimbus is described using 'ISOL' for individual cumulonimbus, 'OCNL' for well-separated cumulonimbus or 'FRQ' for cumulonimbus with little or no separation. 'EMBD' is added to other abbreviations to indicate cumulonimbus clouds that are embedded in layers of other cloud regardless of whether or not they are protruding from the layer.
- Cloud types are given using the abbreviations CU (Cumulus), SC (Stratocumulus), CB (Cumulonimbus), ST (Stratus), AS (Altostratus), AC (Altocumulus) and NS (Nimbostratus).
- When CU (Cumulus) and SC (Stratocumulus) or AC and AS occur together at similar heights, they may be combined; CUSC or ACAS. The cloud base and top are given in feet above MSL.
A slightly obscure place to find "operational" info.