Actually, wouldn't the easiest thing to do if you miss the flight is to stay in CNS an extra day and just call to schedule CNS-MEL for the next day?

I know it's something you don't really want to do, but think of these advantages:

1. Depending on where you start this trip date changes to times (versus locations) might be free.

2. You'd avoid the hassle of reissuing the remainder of the ticket, and you wouldn't have to worry about giving up the seats for the other flights.

3. If you're spending a week on a dive boat and build up a week's worth of nitrogen an extra few hours on the ground (especially since you're flying before, even low altitude) to decomp even with the built in time I'm sure you'll get on the boat isn't the worst thing either before you go up into 1/2 altitude.

Only drawback to this is you'd better make sure of availability beforehand so you don't worry about getting stuck there.

The hoops to reissue especially at a desk likely staffed by people who have no idea how to do an RTW are not insignificant. Counter that with my vague memory of walking up to the counter after I'd missed a flight on my last OWE and rebooked it with a call to the OW desk, and even with the old coupon all went smoothly.

Other than that, scenario 3 is what you'd hope for - my previous RTW would have actually been 21 segments if I'd completed as a forced reissue added one in the US - but sympathy is going to be very important to get this done as unless it's a QF flight don't expect them to do so. Keeping the itinerary intact will cause you a lot less pain and is a lot better option than scenarios 1 & 2.