This gives me the functionality i want:

[ext-did]
include => ext-did-custom
exten => s,1,Set(FROM_DID=s)
exten => s,2,Set(Var_TO=${SIP_HEADER(TO)})
exten => s,3,GotoIf($${Var_TO}" = "<To: <sip:XXXXXXXX@vic.nexvoice.net.au>?ext-local,81,1)
exten => s,n,Goto(ext-local,77,1)
exten => _X.,1,Noop(Catch-All DID Match - Found ${EXTEN} - You probably want a DID for this.)
exten => _X.,n,Goto(ext-did,s,1)

This does not:

[ext-did-custom]
exten => s,1,Set(FROM_DID=s
exten => s,2,Set(Var_TO=${SIP_HEADER(TO)})
exten => s,3,GotoIf($${Var_TO}" = "<To: <sip:XXXXXXXX@vic.nexvoice.net.au>?ext-local,81,1)


[ext-did]
include => ext-did-custom
exten => s,1,Set(FROM_DID=s)
exten => s,n,Goto(ext-local,77,1)
exten => _X.,1,Noop(Catch-All DID Match - Found ${EXTEN} - You probably want a DID for this.)
exten => _X.,n,Goto(ext-did,s,1)