DESPITE favourable firefighting conditions, ACT ESA says firefighters had a challenging night as the Orroral Valley fire continued to expand overnight on the western, southern and north-western edges.

The fire is now about 70,398ha in size.

ACT ESA says erratic and at times dangerous fire conditions, caused by extreme dryness in the environment, were experienced by fire crews on the ground.

But, although it was a challenging night, ACT ESA says crews were able to protect all assets in southern Namadgi including the Corin Ski Resort, Brayshaws Hut, Westermans Hut, Lutons Sheering Shed and Waterholes Hut.

The north-east edge of the fire, closest to Tharwa and suburbs of Canberra remained within containment lines overnight. Firefighters monitored three hot spots on the edge of the fire and reported there was no increase in fire activity.

There are no properties under threat at this time.

The bushfire warning remains at advice.