And the worst weather is still to come later today, NSW Rural Fire Service operational officer for the Southern Tablelands, Lachlan Gilchrist, said.

"The worst conditions will be mid to late afternoon. That's when the temperature will be at its highest and relative humidity at its lowest. Winds are likely to increase, which will make controlling a fire more difficult," Mr Gilchrist said.

"Those hot and dry winds will be in excess of 60 to 70 kilometres per hour and in some places up to 100km/h."

Gusts of 85km/h were recorded at Goulburn Airport and 65km/h at Canberra Airport at 12pm, according to the Bureau of Meteorology.

"Later on today we're expecting a cool change to come through, which might provide some relief to the temperature but with that cool change will be increased wind," Mr Gilchrist said.

The fire at Bowning began around 7.50am and trucks from Bowning, Binalong and Bookham were on scene quickly.

Those brigades contained the fire to a size of about 20 metres by 20 metres but the fire could have grown much larger if started this afternoon, Mr Gilchrist said.

"In an extreme fire danger rating, fires are very difficult to control because they move so quickly."

The crews' fast response was "fantastic" under even "concerning conditions this morning", Mr Gilchrist said.

Mr Gilchrist also thanked the driver who called Triple Zero (000) straight away.

"I suspect we would have been chasing that fire for some time [had it been this afternoon]," Mr Gilchrist said.

"This just highlights to me the need for the community to be hyper-vigilant about fire behaviour in their area. If a fire does start, it will move quickly enough that everyone will need to be aware of everything that's going on so they can make a safe plan."

The cause of the Bowning grass fire is not yet known.

A total fire ban is in place in the Southern Tablelands and a state of emergency has been declared in NSW.

Section 44 of the Rural Fires Act 1997 was also declared around 6pm last night, giving the NSW RFS access to greater resources, Mr Gilchrist said.

Heavy resources will be sent without hesitation to any fires that start today, he said, with the Bowning grass fire an example.

Goulburn SES has six teams on standby to assist the RFS if needed, plus three storm crews and an incident management team in the event of a storm.

NSW RFS Superintendent Peter Alley is also in charge of any fire activity in the Yass Valley, Goulburn Mulwaree, Upper Lachlan Shire and Queanbeyan-Palerang local government areas.