Ambulance NSW's Jackie Levett said there had already been 24 cases of people fainting by late Thursday morning - a higher rate than usual, and most likely due to the heat. People should keep hydrated throughout the day and check up on elderly neighbours, she said. With large crowds expected to cool themselves at Sydney's beaches, paramedics would be watching out for water-related injuries, she said. Extra paramedics have been rostered on ahead of the high temperature forecast. Fire bugs would be shown "no mercy" by police if they were caught, police warned. If they want to behave in a manner where they come to our notice and they start a fire, if through investigation they come to our notice they can expect absolutely no mercy. I mean no mercy," Sydney general metropolitan region commander Assistant Commissioner Mark Murdoch told reporters on Thursday. We will spare no effort or expense to get them before the courts and they can expect to have the book thrown at them.