I think Dale is correct. If he/she isn't then what is the purpose of Australia's ' special Alternate minima' ?
Yep. Alternate and holding is required when forecast, no caveats for if already in flight.
I remember a few years ago arriving into BNE and a TTF was issued indicating vis below the special alternate minima for the duration despite the ATIS and TAF indicating better. After some aircraft began diverting, ATC queried why this was happening. Once they were informed that thanks to the TTF, BNE now required an alternate, a new TTF was issued with the vis slightly above the minimum!