Welcome to the forum and I hope you enjoy your experience as a member of the "Grey Nomads" forum.
We are currently in Canberra and there has been significant rain in and around the "Nations Capital" in the past few days. However the highway to Goulburn is all good. What I suggest you do is call the Police at Goulburn regarding the road that goes Windellama, Nerriga to South Nowra. From my experience the land around Windellama in sections can be very flat and I think prone to low level flooding. But too be sure give the boys/girls in blue a call in Goulburn. Other wise that road is very good and the Naval College at Nowra use that road to bus service people to and fro to Canberra.
The road surface is fine if you go via Tarago and not Braidwood. I believe there is still some gravel between Braidwood and Nerriga.
All travellers in NSW who are travelling known usually good roads should consult this RMS site. It is updated far more often than any other site. They do it 24/7.
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PeterD Nissan Navara D23 diesel auto, Spaceland pop-top Retired radio and electronics technician. NSW Central Coast.
I have done this trip several times
There is a section that is steep and windy however this section maybe less than a 1km
I put these waypoints in my GPS
Bungadore, Tarago, Nerriga, Nowra
I don't think it would be any harder than Kangaroo Valley
Chris