FIRES have threatened heavily populated areas in Sydney and caused smoke to billow over a busy motorway. An emergency warning for residents living along four streets at Kemps Creek, near Liverpool in the city's southwest, was issued just after lunchtime on Friday as a grass fire made a run towards people's back fences. More than 250 firefighters and two air cranes worked to get the blaze under control with a number of residents choosing to leave. Smoke from the blaze began blowing over the M7, prompting the Rural Fire Service (RFS) to tell people to avoid the area as much as possible. But the blaze was brought under control by the early evening after burning through 250 hectares. "Firefighters continue to patrol at this stage to make sure it doesn't flare up again," RFS spokeswoman Natalie Sanders told AAP. Meanwhile, two blazes in North Epping and Pymble in Sydney's north were believed to have been the work of arsonists. "We will be investigating these in the coming days," Ms Sanders said. It came as Sydneysiders awoke on Friday to hazy skies, with smoke from a fire in the Hawkesbury region which has already burnt more than 35,000 hectares making its way over the city. Extreme conditions that were forecast for the state also prompted a total fire ban and the closure of all bushwalking tracks in national parks across greater Sydney. Ms Sanders said while Sydney was expected to experience benign fire weather on Saturday, conditions in the Hunter region were forecast to be severe. With gusts of up to 40 km/h and temperatures in the high 30s, she said the RFS would turn its attention to that part of the state. "We are ready to respond to that as required," she said.