I just did Canberra-Gundaghai-Tumut-Cooma-home today just to try a 500km day in 32c heat (as you do!) The road is in good condition, but watch out for the narrow windy section around Blowering Dam as there is loose gravel over the bitumen for 5kms. It is deliberate, and slippery stuff - you even have to stick to the 60km speed limit... The slippery bit satrs 100m before the warning sign, as cars and trucks mut have dragged gravel down the road. I was too busy correcting a rear wheel slide to see if the traction control did the work or me.
Just south of Cooma there is road resurfacing going on just out of town before the Tumut turnoff. Delays will probably be over before the weekend.