Three girls will be nominated for bravery awards after they helped to evacuate a heavily pregnant woman and her young children from a burning unit block in Sydney's west.

Police said the girls, aged 12, 13 and 15, showed remarkable initiative and courage by alerting the woman who was still inside her unit in Blackett as the unit next door to hers went up in flames.

The girls - Tiarnna Bevege, her stepsister Brittany and their friend Jessica - were walking to a friend's place about 5pm on Tuesday when they saw thick smoke billowing from a unit on Popondetta Road.

"We seen lots of black smoke, so we ran over to it and we seen the lady's house on fire and there was a man that was already putting the lady's house out, and he yelled out to go get [the] kids in number two, so we ran to the house," Tiarnna told radio station 2UE.