Authorities are expecting the worst as 68 wildfires continue to burn in Australia - 22 of them out of control - and dangerous weather conditions forecast through Thursday.
"There will be a significant impact on populated areas should all these forecasts materialise as is being suggested," Shane Fitzsimmons, New South Wales Rural Fire Service Commissioner, told reporters.
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The suspect, 35-year-old Willie Corey Godbolt, has been arrested and was being treated for a gunshot wound in hospital.
Mohanna Abdhou, known as Montana, was out with friends when two attackers cycled up to them and opened fire in Brent, north-west London.
The US president tweeted that Mrs May gave him "full details" of the incident during talks while he attended summits in Europe.