I have no idea, but CASA doesn't seem to think it's a 'grey area'. From the ATSB report:
"CASA clarified that there was no ‘strategic difference between an in-flight scenario and a pre-flight plan’ in relation to the use of forecasts.
In relation to the use of observation reports for in-flight planning, CASA noted that ‘weather observations are not a legal instrument to determine if an alternate should be held or for fuel planning, unless the observation has a trend appended to it (eg TTF…)…’."