So Phil, trying to think a bit like an Irishman.
All those numbers where they are costs are in Euros.
Heres a conversion siteBut to keep it simple, add on 30% for US$
You're skimping on Thailand but Vietnam and Cambodia could be a bit less and will roughly be similar to Thailand so the Asian curry could come out mildly right.
There's a thread from earlier this year, probably within the last ten pages that has some pretty good breakdown and views on cost in the different asian countries - it had about 50 or so responses and soshould be easy enough for you to pick up - worth having a look.
You've allowed a reasonable ammount for Oz (if not time) and NZ, but costs can go high pretty quick depending on what activities you might want to do.
Have a look at the Australasia & PI forum, again going back five or six pages and you'll find all sorts of references to doing things on the cheap - and in particular for Oz., with limited time, the more you think of your plans for there and maybe seek a few cheap airfares in advance, the better you and your budget will be for it.
As for arriving in Thailand in October and doing SEA, there was significant flooding in parts this year and generally you will be near the peak of the wet season.
You might appreciate it all more if you move your departure back a couple of months and that way you will be getting to Oz when it is still hot enough to fry an Irishman but you will at least have missed the summer holiday crowds which can make getting accomodation an issue.
NZ weather will still be not too bad (hopefully)(bit like Ireland in parts) and though it would start to get cooler in southern parts of Chile/Argentina, it will probably be better in heading north.
You didn't leave 17 coasters lying around in a pub with all those numbers on by any chance did you - you could have added names and there could have been some really Irish calls going down!
Flights from NZ to SA can be expensive, so I'd keep my peepers on the lookout - might even be cheaper to get a return ticket and ditch the return.
If I haven't turned you on your head enough, have you considered if sticking with existing timing going the opposite way around!
Ireland - (US)for cost - SA - NZ - Oz - Asia
As that would have you getting good weather going down SA and in Oz and NZ, be hot in Asia (but it don't vary all that much in equatorial areas)but you'll probably miss most of the wet and China will be good weather instead of getting a bit cool if you plan on doing it in November/December sometime as per your current itinerary.
If you plan on leaving or arriving into HK, there's extra good fares with
www.oasis-air.com , and thus keeping with your existing timing and direction, if you fly into HK, do China and then SEA, that'll give you a good chance of missing a lot of the wet.
And every chance you'll use up your reserves but that's what they are for, even in Rugby.