Information available here:
AS2118.1 1999 Clause 5.4.8 – Clear space below sprinklers
Except as provided in Clauses 11.1.3.4(b) and 11.1.3.6(d) a clear space not less than 500 mm shall always be maintained below the level of the sprinkler deflectors throughout the room. For high piled combustible stock, clearance not less than 1m shall be provided. Roof trusses shall at all times be accessible to water discharged from the sprinklers.