The Mary's Hill fire burning to the west of the ACT grew slightly overnight on Tuesday, but remained inside containment lines on Wednesday afternoon.

Emergency Services Commissioner Georgeina Whelan said that while rain would be welcomed, consistent rain over four to five days was needed to assist the firefighting efforts.

"We don't want a really heavy downpour for half an hour that creates flash flooding," Commssioner Whelan said.

"It also does not lend itself to safe fireground for firefighters to attack fire, vehicles can be bogged."

More than 50 millimetres of rain over a period of days would "buy us time" Commissioner Whelan said, but would be unlikely to fully extinguish three blazes that burn just outside the ACT.

A sudden heavy downpour could even lead to firefighters being removed from the fireground if it was deemed unsafe.

"Firefighter safety is our number one priority. A combination of the pre-positioning we have of our vehicles and also our own aviation assets and the ADF assets have allowed us to keep our firefighters at the moment safely on the fireground with the ability to extract, based on not just the fire activity but also the weather."

Defence Force aircraft could fly in more challenging conditions, Commissioner Whelan said.

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"[Firefighters] would also be pulled out for heavy rains for a short period of time. What you may see is a very short, what we call, an operational pause in firefighting and that would be for firefighter safety."

While Chief Minister Andrew Barr said the Mary's Hill fire was causing the most concern for the territory's firefighters, Commissioner Whelan said it was still inside multiple layers of containment lines.

On Wednesday it was burning 4.8 kilometres from the ACT border, and Commissioner Whelan said if the weather remained as predicted, it would be unlikely for the fire to cross the border.

Emergency Services had undertaken significant work to protect Pryor's Hut on the outskirts of the ACT, with areas of bare earth established around the historic hut, as well as installing hoses and grip systems and a water supply for those systems.

While much of the containment work to stall the spread of the fires is being done with heavy plant equipment, flora and fauna were also being protected.

"They call it strategic engineering, precision plant activity," Commissioner Whelan said.

"It is almost like a surgical movement out there, in terms of the focus they have on value-adding to the very good work we've done ... in terms of respecting our landscape and the flora and fauna that makes Canberra such a fantastic place to live."