Police are looking for a St John medical bag that disappeared from an ambulance while staff treated a seriously injured teenager.

St John staff were called to Cross St, Papakura, yesterday about 8pm to treat a 13-year-old boy who fell off a bicycle.

The scene was very volatile, and a number of people were heavily intoxicated, said prevention manager Acting Inspector Matt Hoyes.

The patient and others at the address were reportedly aggressive to ambulance staff.

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Ambulance staff left quickly taking the patient to hospital, but did not realise a medical supplies bag was missing.

"It is unclear at this stage whether the bag has been taken while ambulance staff were at the scene or inadvertently left behind by St John staff," Hoyes said.

The ambulance staff went back to the scene with police but could not find the bag.

Police ask anyone with information to contact Detective Sergeant Steve Williamson at Papakura Police Station on (09) 261 1300.

Or, people can contact Crimestoppers anonymously on 0800 555 111.

Police were not making any further comment at this stage.