Two females who drank an 'unknown substance' have been flown to hospital.

Ambulance and the Westpac Rescue Helicopter were alerted to the situation at Waihi Beach just before 8pm. At the popular holiday spot, two female patients had medical complaints after drinking the unknown substance.

They were airlifted 70km to Waikato Hospital in a serious condition.

An ambulance spokeswoman said the substance still hadn't been identified

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"Both patients were taken to Waikato Hospital. We still don't know what the substance was," an ambulance spokeswoman said.

The spokeswoman said the age of the patients was not known. Police did not attend.