So I was wondering where people acquire gear for larger sizes?
I'm an incurable documenter (documentor?) so I can't help making lists of whatever I've learned so far. So here are a few places I've found, mostly by hunting around online and asking other larger sized outdoorsy people, but I must admit I haven't ordered from all of them yet.
For clothing, the following online retailers have outdoor gear or stuff that's outdoors-y up to quite large sizes (all measurements given are bust/chest measurements):
Columbia (http://columbia.com) -- primarily an outdoor retailer so they have just about anything you'd want eg. zip-off trousers, UV-rated shirts, rain gear, etc. Women's and men's sizing to 53" (chest/bust) but larger sizes often run small according to what the website says. I'm told Ray's Outdoor stock these but I have never seen any there in decent sizes. DStore seems to sell Columbia (including larger sizes) online in Australia: https://dstore.com.au/brands/columbia/
LL Bean (http://llbean.com) -- general clothing store tending outdoors/active; base layers, rain gear, etc etc. They have a plus size range (to 53") but much of their outdoors gear isn't available in this sizing. However they have some long underwear/baselayers (especially silk) and rain gear at present and they seem worth checking.
Lands End (http://landsend.com) -- general clothing store that carries a lot of fleece. Tends to be a little bit daggy but there are some gems to be found. Plus sizes to 54"/140cm (men's to 56"/142cm). Check the sales/clearance areas for some good deals. Return shipping available to an Australian address.
Eddie Bauer (http://eddiebauer.com) -- general-tending-outdoorsy clothing store, some fleece, activewear, and rain gear, a few performance fabrics etc. Women's plus sizes to 54", men's larger sizes to 56". Check the sales/clearance.
Terramar -- makes base layers and stuff, women's sizes to 51.5". Available via various online vendors, mostly US based. Might be hard to find someone who'll ship to Australia; I haven't tried.
REI.com -- their own line of clothing has women's plus size to 50", and they also stock some other brands eg. Columbia in a wide range of sizes; expensive shipping from the US ($40 on orders $50-$100).
White Sierra -- women's plus size to 53"; they won't ship to Australia direct but are available via some third-party vendors (eg. REI.com).
Woolovers (http://woolovers.com.au) -- specialises in merino and other natural fibre basics. Not specifically outdoors nor plus sized but you can find things like lightweight merino jumpers really cheaply and though the size chart may say 44" or something, if you're wearing them as a base layer they're pretty stretchy. (Note: their sizing varies from garment to garment so check the size chart for EACH thing you look at!) Lots of unisex/men's options too which of course have a longer length in body and sleeves (would work for some women but not others, and sadly not me). Ships from UK but return shipping to an Australian address is available.
And a local tip or two...
Target Australia have merino long johns (tops and bottoms) very cheaply, up to size 20 which they claim is to fit around bust 120cm/hips 125cm but of course they're stretchy and will go a bit bigger. They're usually available in black, charcoal, and one special colour each year (this year seems to be an eggplant/grey stripe) are $30 a piece but wait until Target have an underwear sale

The Cancer Council's online shop has UV-resistant/breathable summer shirts and other stuff up to about a women's size 22/118cm or men's 4XL/125cm.
Other thoughts:
I haven't been able to find a lightweight/packable rain coat that I like and fit into (let alone pants, sigh) so I'm going to try a poncho when I decide I'm over carrying my current bulky raincoat. Sea to Summit make what appears to be more or less the standard, in two models (standard nylon and sil-nylon, about $60 and $120 respectively), readily available all over the place.
Anyone got any other tips?