When Geoff Keaton and his volunteer firefighting friend Andrew O'Dwyer became fathers within two days of each other in May 2018, life couldn't have been more promising.

On Thursday night, that promise was destroyed near Buxton, south of Sydney, when the two were killed trying to contain the Green Wattle Creek blaze, becoming the first volunteer firefighters to die in the fires that have devastated nearly 4 million hectares of NSW since July.

On Friday morning, Australians were in mourning for Mr O'Dwyer, 36, who ran his own photography company, and Mr Keaton, 32, whose father John, a fellow firefighter was out fighting the Gospers Mountain fire in the Blue Mountains.

Flags flew at half-mast across NSW, including on Sydney's Harbour Bridge, in honour of the fathers whose deaths have taken the fire toll to eight since November.