A volunteer firefighter is in a serious but stable condition in a Sydney hospital after burns to his face and hands from the blaze off the Hume Highway near Yass on Sunday.

NSW Rural Fire Service spokesman Joel Kershaw said the man was not on duty when he was burnt.

"He wasn't with the brigade and on a truck, he was working in a private capacity, and suffered burns to his hands and face as a result of the grassfire," Mr Kershaw said.

"I can't give a name and age at this time as there is still an [Rural Fire Service] investigation under way."

The man was flown by helicopter to Royal North Shore Hospital on Sunday night.

The fire started about 5pm and burnt 140 hectares near Bowning, north-west of Yass, before being declared out at midnight following the work of more than 110 firefighters.

A separate Rural Fire Service spokesman said the fire caused the death of 20 sheep on one property and six cattle on another.

An unused house and sheds used for storage, as well as a shearing shed, were destroyed on a third property.

Sections of the Hume Highway and Lachlan Valley Way were shut for several hours.

In the ACT, the northbound lanes of the Monaro Highway at Tuggeranong were shut for about an hour on Monday after a small grassfire.

Emergency authorities received multiple calls about the fire at Theodore from 1.30pm, with flames reaching two metres before being contained within 25 minutes.

Smoke caused the highway to be closed in both directions at Johnson Drive, with southbound lanes reopened about 2pm and northbound lanes open about an hour later.

An area of about 2500 square metres was burnt, with four units and two commanders initially at the scene. The cause of both fires is being investigated.

Mr Kershaw gave a number of tips for drivers who spotted fires this summer.

"We encourage people to report them, even if they feel it may have been reported to triple-zero already," he said. "Obviously make observations of your surroundings, slow down, and take advice from emergency services.''