Actually nothing changes the ratio. The ADRs are a security with a fixed ratio. Each ADR issued by the US repository who administers the SSN ADRs must be backed by 20 shares held on their behalf by the Australian nominee.
If options get exercised there are simply more shares on issue. The total pool of shares simply get bigger. The nominee can buy more shares on market here in AU allowing the depository to issue more ADRs but each new ADR must be back by 20 SSN shares in the nominee account as per all previous ADRs issued. The ratio never changes unless there is a capital reconstruction.
The exchange rate doesn't impact the ratio either. Neither the value of the AUD or the USD has any impact on the 20:1 ratio of SSN shares that backs each ADR.
Remember that a share represents a faction of ownership of the company. ADRs are merely a receipt promising the receipt holders title to 20 SSN shares. If the pool of shares (including the ones backing the ADRs) gets bigger then the percentage of ownership implied by each share gets slightly smaller. The value implied into share prices and the implied value of a current by virtue of exchange rates are completely different to the underlying legal entitlement of the shares themselves.
SSN Price at posting:
19.0¢ Sentiment: Buy Disclosure: Held
