The value and quality of the grain only makes a margin...

The value and quality of the grain only makes a margin difference to GNC, this is based off higher quality meaning you have a more front end loaded export program (less storage rev for GNC) vs more tonnes go into their system rather than being stored on farm. Feed wheat will take longer to export (more storage rev) but farmers will more likely store on farm thinking that it'll eventually go into the domestic market.
The idea that 1.5t/ha or even 1.5Mt was aggregate lost over the weekend is a massive stretch and certainly more than anyone I know is quoting. Crops in SNSW/VIC benefited from the rain so even the isolated damage in NNSW is more than offset by gains down south. The downside of feed for GNC is it takes the weight out of the crop so your test weight gets hit and they bill on weight (i.e. $ per tonne) so if you're taking the same amount of trucks but the grain doesnt weight as much = lower S&H $$.

The truest way of establishing whether there is big downgrades in quality of wheat is looking at the price of feed wheat yesterday which was unchanged from the week prior, telling you the mkt didn't view the weekends rains as enough for a large scale feed downgrade (which is inline with all the growers I've spoken to). Another rain or two like that in NNSW will start to throw up some problems on quality in the areas that got the most rain over the weekend