Lansbury levelled the scores just after half-time, with Forest going in at the interval trailing to Andrew Crofts' header just after the half hour mark.
Two goals in three minutes put Forest in the driving seat, with Lansbury slotting home from the spot almost immediately after Darius Henderson had given them the lead before Nathaniel Chalobah was sent off in the 67th minute as Forest finished the match with 10 men.
Brighton went into the game having scored only four goals in their previous five matches and, in a lively start, former Newcastle winger Kazenga LuaLua provided an immediate threat for the Seagulls.
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Wednesday morning's weather for the east of the region
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More than 30 huts have been burgled along Herne Bay pier, in what seems to be a systematic attack