thanks.
I am not a tax agent and bar far not an expert however I have a theory on this.
Since no government (I think) has acknowledged Bitcoin as currency, so long as you hold it in BTC, it essentially does not exist. When you cash it out, it's like receiving a lump sum cash payment form someone, which would need to be declared as income.
So at worse you would only pay tax once, unless you declare the BTC at it's fiat value as capital and have to pay capital gains tax.
Lots of q's around this obviously but with the right accountant you should be able to limit to paying tax once.