There are, in my view, two ways to approach this; a strict view on the contract and a broader “first principles” review.
Under a strict contract view presumably they value at nil because the first part of Equipment in the SCC says that it relates only to Equipment used within the WA and as your equipment wasn’t it then doesn’t fall neatly under any heading of the SCC. If it is not in the SCC (or indeed the SSCC) then it cannot be recovered. In holding the equipment ready you took the risk, notwithstanding your EWN etc.
On a “first principles” review, subject to a requirement to mitigate (ie off hire or reallocate) yes this cost should be recoverable. There may be an evidential issue with demonstrating that the equipment was held specifically for the project and was therefore able to mobilise immediately but you seem to have that covered with the EWN you menti8on. There may also be some discussion around how long you should hold it rather than reallocate or off-hire (ie mitigate) but given your notices it seems unlikely that figure should be nil.
I don’t think there is a clear right answer here to satisfy both first principles and the contract. However, on balance, I think the first principles approach is to be favoured where you gave EWN stating equipment was being held off site and otherwise complied with the contract. There sounds to be a tacit agreement to holding the equipment off site which should lead to payment, ie the first principles approach.