Bushwalk Australia

Title says all. Whats light, cops the *&%$#! and the cheapest. Now I know i,m dreaming. for use under a tarp. Simple corpse bag type.

Pick two of those wants/needs
Evans Tyvek cover but use a groundsheet

That unfortunate "pick any two" comes in to play with most questions like this.
You can get a very breathable and light and sort of cheap Tyvek 1443R type but I would not call it durable and in winter I imagine would not be that much of help with heat retention.
(I have made one for myself but not used it on snow or cold weather because it does not fit in with the shelter I use)
You can get a very strong and durable GoreTex type that can be cheap ex military but not light.
eVent is lighter , breaths well but not for summer use (if you need it to be bug proof) and not cheap.
Stuff made with 10-15d fabrics can be very breathable but again not all that cheap nor durable if you are just a little bit rough with it.
Long comment to say I haven't come across one that is close enough to what you want.
( I suspect that most bivy users would like one of those....)

Thanks fellas, you've answered what I thought! ok next one: best way to reproof a gortex top/nylon bottom old bivi bag?

go to an outdoors shop and get detergent suitable for using with Gore tex, and a DWR treatment,
grangers or nikwax are two brands that make these

recently used nikwax on the bathtub floor of a megamid and it didnt cut it (i was on the bank of a flooding river). thinking of doing a Franco style syl nylon coat on the bottom and then nikwaxing, anyone done this?

If the Megamid floor was PU coated nylon (probably was) I would use Tent Sure.
http://www.wildearth.com.au/buy/gear-ai ... t/MCN90021

thanks franco, had a very unhappy tent mate on the arthurs recently.

Going back to the military style bags in goretex for a second....

I've got a couple of the U.S.version and yes the weight is an issue but! they are massive in size as well, designed for 2 BIG sleeping bags+ beefy sized soldiers dressed, boots and all.
Consider grabbing one and trimming it down to size a little? I'm 5'9" and I reckon I could lop 30cm off the end and 10-15 down one side and it would still be more than roomy. Mine measure 2.1m x 780mm and both the warm+cold bags slide in easy [ then it's an oven!]. So a trimmed down bag, light insulated pad and down bag/quilt would be very usuable/rugged.

Cheap to buy, extremely robust, zips and press studs super strong, something to consider if you want "bombproof" [ bad pun, sorry ]

The MSS has its good points, mainly how cheap they are as surplus
All of the above apply, strong, heavy, durable, heavy, robust, heavy and overkill for bushwaking. Did I mention they were heavy?
I use mine for car camping and at my base camps as it weighs 1600 grams
Very difficult to cut down/make smaller tho
Evans Tyvek bags are sleeping bag protectors really but in most instances that is all you need.
Nikwax or silicon but not both

Turf and buy lightweight tent. Much more user friendly. You can even almost sit up in them!

Andrew for nearly all purpose I agree. I have a nostalgic view and when it goes tits up and you crawl into the body bag ànd all is good, well if ya know it