Firefighters and police are praising the actions of three teenage girls who rescued a heavily pregnant woman and her three children during a unit block fire in Sydney's west.

The girls, aged 12, 13 and 15, were walking on Popondetta Road at Blackett about 5:00pm (AEST) on Tuesday when they noticed the fire.

A woman, who was 40 weeks pregnant, and her three young children were inside a neighbouring unit.

Two of the girls helped the woman and children from their unit, while the other girl called emergency services.

Tiarnna Bevege says she and her friend Jessica went inside the house to get the children, aged one, three and five.

"It was very quick, we just got the kids and then got out, we didn't want to stay in there," she said.

Mother Melinda Bevege says she is very proud of her daughters' actions.

"Until we came here today I didn't realise the extent of it," she said.

"They're brave. Really, really brave. It's selfless it's just pure instinct for the girls to run in there and help other people.

"We're taking them out for dinner tonight."

Inspector Steve Moran from Fire and Rescue says their actions were extraordinary.

"To think that they'd be switched on enough to do that without any protective equipment is just an amazing thing," he said.

Fire crews extinguished the blaze and the pregnant woman was checked at the scene.

Another woman who was found in the back yard was taken to hospital for assessment.

Inspector Bill Pearce says police intend to nominate the girls for bravery awards.

"They obviously are sensible and intelligent young women and they obviously saw what was going on and realised that there was a need for someone to help," he said.

"They were asked to help and without hesitation, and very bravely, they've entered the premises and assisted the lady."

Police from Mt Druitt Local Area Command are investigating the cause of the fire.