A bushfire has broken out at a popular Perth wetland, despite the city enjoying a burst of cool autumn weather.The fire is burning on the eastern side of Herdsman Lake, 6 kilometres from the Perth CBD. It was reported at about 10.30am on Monday and is moving in an easterly direction through inaccessible bushland.An advice alert is in place for people near Herdsman Lake in the suburbs of Glendalough, Osborne Park and Wembley.Of most concern are embers being blown around into the residential areas surrounding the lake.Authorities say the fire has burnt about 20 hectares and is contained but not controlled.There is a lot of smoke in the area.The Department of Fire and Emergency Services (DFES) said the fire started within the swamp area and crept to the edges, with tall reeds increasing the flame height.There is no access to the lake itself, so fire crews had to wait for the fire to reach the lake edge to stop it from spreading further, according to DEFS. Crews were also assisting from the air, it said. The temperature in Perth today is expected to reach 23 degrees, with the city's fire danger rating varying from no rating at all, to moderate in some parts.Herdsman Lake is known for its walking trails and dense flora as an inner-city wetland.It is also known for its large population of birds and tiger snakes.