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The home side's dominant win guarantees them a play-off berth and means the semi-finalists have been found one round early.
They will join Taranaki, Canterbury and North Harbour regardless of the results in the final round next weekend.
More interest will focus on the fate of Auckland, the most successful union in NPC history with 16 titles, whose final match is against powerhouses Canterbury.
They are locked in a dogfight with another proud province, Waikato, to avoid dropping to the second-tier Championship.
Waikato are three points behind but have two matches remaining, against North Harbour and Bay of Plenty.
Auckland's sixth loss from nine matches came via an error-strewn effort at a waterlogged Trafalgar Park, where they were outscored five tries to two.
Tasman dominated all facets to become the first team this season to win both games in a gruelling Wednesday-Sunday double.
Winger James Lowe and 19-year-old fullback Will Jordan defied the conditions with their speed and skill, having a hand in most of the tries.
Two early tries came through slick team moves, with halfback Billy Guyton crossing in his 50th appearance.
They led 19-6 at halftime, helped by a crucial tackle from Tasman back Trael Joass on his All Blacks Sevens team-mate Joe Ravouvou to deny the winger what seemed a certain try.
Jordan bagged his fifth try of the season after the break and lock Shannon Frizell responded to Auckland's opening try through Taleni Seu to virtually secure the result with a quarter to play.