Because the funds of a banker's draft have already been transferred they are proven to be available; unless the draft is a forgery or stolen, or the bank issuing the draft goes out of business before the draft is deposited and cleared, the draft will be honoured. There is a caveat in that the bank protocols may declare the draft too old to be valid; this has been known to happen when the draft is over six years old.
Since the funds of a banker's draft have been transferred, they are proven to be available; unless the draft is a fake or stolen, or the issuing bank becomes insolvent before the draft is deposited and cleared. Additionally, there is an exception that the bank may consider the draft too old to be valid, which has occurred when the draft exceeds six years.