A police investigation into a birthday cake laced with cannabis, which was served at a Nelson restaurant to unsuspecting diners, has been inconclusive.

The cake, made at Nelson's The Waka restaurant, was served to a group at a birthday dinner on July 3, Fairfax reported.

One diner, who consumed the cake, told Fairfax it tasted "weird" and her husband became jittery afterwards.

About six people were reportedly treated at Nelson Hospital after eating the cake.

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A joint Nelson Marlborough District Health Board and Ministry for Primary Industries (MPI) investigation detected the presence of cannabis, and referred the matter to police, Fairfax reported.

Police said that though the investigation was complete, there was insufficient evidence to prove the cake was the source of the cannabis.

"While Police believe it is highly likely that these people were served a cake that was laced with cannabis, there is insufficient evidence to prove this," sergeant Sally McBride said.

"The people involved underwent tests, which confirmed the presence of cannabis. Unfortunately, at the time of reporting no cake was seized for examination."

The restaurant's owner told the Nelson Mail the allegations were not true, but that he would now freeze a sample of every cake made by the restaurant as a preventative measure.