The Bureau of Meteorology says the national mean temperature of 22.76 degrees was almost one degree above the long-term average.
Queensland and New South Wales experienced their warmest years since records first were kept in 1910.
Prolonged high sea-surface temperatures also led to the second consecutive year of mass coral-bleaching on the Great Barrier Reef.
The head of climate monitoring at the Bureau of Meteorology, Dr Karl Braganza, has told SBS it was the first back-to-back summer bleaching event on record.
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