A try from fly-half Jonathan Sexton - and another Leigh Halfpenny conversion - put the Lions clear again heading into the final quarter.
Sexton touched down after receiving the final pass from Halfpenny in a slick Lions move in the left corner.
Australia had closed the gap to just three points early in the second half through two Christian Leali'ifano penalties.
Halfpenny added another penalty of his own.
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Wales assistant coach Robin McBryde says it's all about momentum ahead of this weekend's Pool A decider against Australia.
Wales still have it all to play for against Australia at Twickenham this weekend. The prize - an easier route to the World Cup finals.