The talk before the weekend in Melbourne had been about the step forward made by Ferrari over the winter but their fastest driver Kimi Raikkonen was down in fifth.
Hamilton was using faster tyres when he set his fastest time, but the Mercedes appeared quicker on the same tyres, too.
Hamilton's team-mate Valtteri Bottas was second ahead of the two Red Bulls.
More to follow
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Australian Grand Prix first practice results
Australian Grand Prix coverage details
Lewis Hamilton led a Mercedes one-two in first practice as a new era of Formula 1 started in familiar fashion at the Australian Grand Prix.