 As it entered the area around Melbourne, it came across a heavy cloud layer, extending from 1500 feet (457 m) to 400 feet (122 m) and making landmark navigation difficult.As a result, the flight crew mistakenly identified Sunbury as Daylesford through a gap in the clouds, leading them to believe that they were 30 kilometres (19 miles) behind where they actually were on their flight plan.Had the flight crew cross-referenced their ground speed with previous landmarks, they would likely have realised that they were not where they thought they were.