1. If I decide to claim my BIG shares as capital losses, do I need to do so before the company publicly is delisted or do I have a chance to see if BIG relists?
If you are not facing a capital gain for this FY, and therefore do not need the capital losses from BIG to offset your gain, then there is no rush to do anything. The company will either get wound up later and you will get your capital loss then, or it will get recapitalised and relist. If it relists, you will probably be diluted to worthlessness anyway and you will be stuck with an unmarketable parcel of shares.
On the other hand if you are facing a capital gain this FY, you may want to consider getting rid of your shares now to offset that gain (this is to avoid the potential perverse situation where you have to pay a crapload of tax this FY only to be up for a huge capital loss next FY that you won't be able to use up very quickly). You can't sell BIG right now because it is suspended but www.delisted.com.au provides a service that enables you to crystallise your losses this FY by basically giving it away. You will get zero for giving the shares to delisted.com.au and in fact, you will need to pay a small fee for them to facilitate this process. Contact delisted.com.au for more info.
2. If I do claim this loss, is the total value of the loss counting to reduce my tax or only a portion of my loss?
Your total losses on BIG can be used to offset other capital losses you've made this FY. Any unused capital losses can be carried forward to future FY's. This a very simplistic explanation however and I strongly suggest you see the info provided on the ato website and rely on what they say. e.g. https://www.ato.gov.au/forms/guide-to-capital-gains-tax-2017/
Disclaimer: This is just my opinion, and is not personal advice as I have no idea about your personal circumstances. I have not used delisted.com.au and cannot vouch for it personally. Although I think it is extremely unlikely, it's just IMO and there is obviously a chance BIG may relist and there may be some value however, it is likely it will be very small and almost worthless anyway.
The only advice I will offer is not to rely solely on HC for financial decisions. Contact the ATO and your accountant/financial adviser/lawyer for more info and advice, and DYOR.
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