New Zealand police said Thursday they have recovered 29 methamphetamine tablets disguised as pineapple sweets that were mistakenly handed out to the public in charity packs.

Police are racing to track down more of the brightly wrapped drugs, which were anonymously donated to anti-poverty charity Auckland City Mission and unwittingly redistributed.

In tests, one of the pieces was found to contain approximately three grams of meth -- a potentially lethal dose of the highly-addictive drug.

A criminal investigation has been launched as police try to track down up to 400 people who may have received the fake sweets.

"Police will work quickly to secure any of the items out in the community, but equally, part of the puzzle is working out how many may still be out there," Detective Inspector Glenn Baldwin said in a statement.

Four people -- two children, a teenager and a charity worker -- have so far been given medical treatment after tasting the sweets, police said. None are seriously ill.

Police are also investigating a claim that one of the wrapped candies was up for sale on Facebook Marketplace.

Police believe the drugs might have been packaged inside the distinctive yellow "Rinda" wrapping by traffickers or dealers to evade detection.

Malaysian candy manufacturer Rinda Food Industries told AFP its branding had been "misused" and it did not "condone the use of any illegal drugs in our products".

A spokeswoman for Auckland City Mission has said the charity was "devastated" by the incident.