The robber was given some cash before he fled the scene in the Hutt City suburb.
Police today released security camera footage they hope will help identify the offender.
Detective Fleur de Bes, of Lower Hutt CIB, said the "very cool and collected" masked man strode purposely into the TAB in The Strand at 11.10am, threatened a staff member with a knife and left with less than $300.
The man, described as Maori and tall, wore a plain grey hooded sweatshirt, a black balaclava, dark track paints with white trim, white trainers and white gardening gloves or similar.
He fled the TAB and was last seen turning left into the direction of Wainuiomata Road.
"Where he went then is not known," Ms de Bes said.
"It's possible he may have gone into nearby parks."
Several TAB patrons were inside the premises while the robbery took place.
"Most didn't realise the robbery was happening until it was all over as they were organising their bets and watching TV screens," she said.
Ms de Bes praised TAB staff for remaining calm and complying with the robber's demands.
