Slang for MDMA. "Kathie" is a powder form of the drug ecstacy. Is often preferred to ecstasy pills as MDMA is pure, whilst ecstasy pills are often adulterated. It is a popular drug amongst ravers and people who do not trust the purity of pills, and ecstasy in general has always been associated, often by the police and government, with the underground dance music scene.
Other slang used "Mud"
"Alrite mate, you got any Kathie for the mash-up tonight? If not I'll get some beans (pills)"