Their leisurely picnic on the river could have turned to disaster were it not for three bars of mobile reception, a quick-thinking Triple-0 operator and the Emergency + app, Loren Madrid and Anna Skaljac said.

The 19-year-olds were among a group of four teenagers firefighters rescued from Pierces Creek State Forest on Sunday night, after their Holden Commodore sedan became stuck on a four-wheel-drive track at Murrays Corner about 6pm.

"We were driving off one of the roads and we saw this steep hill and we thought that was an exit. It obviously was not because at the end of the hill we couldn't get back up, we just didn't have enough power to get up, it was so steep," Ms Skaljac said.

Fighting fading light and plunging temperatures, they tried to push the car up the sharp, muddy embankment for about half an hour before giving up.

While they had brought food and water with them, their mobile phones were almost flat and nobody had any idea where they were.

With 34 per cent battery on her phone, Ms Madrid broke away from the group to search out a decent mobile signal.

"I was able to get 3G out there and I searched who I should I call. I called the SES first and they said wrong line so they put me through to community services who said call triple-0," she said.

It was firefighter Kaye Bradtke who picked up the call and advised a panicked Ms Madrid to download the Emergency + app if she could.

The app displays the GPS coordinates of the phone's location, which a caller can read to the emergency operator.

"Twenty seconds later and it was on there. I gave her the coordinates and she said we've got someone coming directly to you guys," she said.

An hour later, the teens saw headlights pierce through the trees and heard the sirens coming for them.

Once they arrived it took another two hours for firefighters to winch the Commodore out.

ACT Fire and Rescue commander Steve Gibbs said it was the first time in the ACT this app had been used this way.

"Without [the app], it would have taken until daylight [to find them], especially out in the forest where they were. The Brindabellas is a very big area and our view is they would have spent a very wet, cold night in their car and we would have had to launch a major search for them in the morning," he said.

But he warned teenagers that just having the app handy might not be enough to help in an emergency and to always let somebody know where they were going.

"They were very lucky they even had mobile phone coverage, without mobile phone coverage - and it's very spotty in the Brindabellas - we wouldn't have even known they were there," he said.