Early risers along the New South Wales coast may have been surprised to see big waves as the sun rose this morning. A large and powerful groundswell originating from a system traveling to the south of New Zealand arrived on Australia's east coast a bit earlier than expected. Waves at some of Sydney's most popular beaches were reaching double overhead this morning, or about twice the height of an average person. While computer models had predicted the swell to arrive later today, this morning's first light showed that it had turned up overnight. Data recorded by Sydney's waverider buoy showed that maximum wave heights climbed by 2.5 metres overnight and the peak wave period, which measures the power of the swell, has doubled to near 20 seconds in the last 24 hours. This long-length wave period is rarely seen on the NSW coast and is more typical for coastlines that are exposed to broad expanses of ocean, like Hawaii or southwest facing coastlines of southern Australia. These longer period swell travel more quickly than lower period swells and maintain their size and energy for many thousands of kilometres if uninterrupted by land. Conditions will be dangerous for unsuspecting surfers and swimmers visiting the NSW coast today. So, it's a good idea to avoid activities like surfing, swimming and rock fishing.