West Coast Police have been called in the investigate a case of milk spiking.

Police say Penicillin was deliberately dropped into a tank of fresh milk on a West Coast dairy farm near Inangahua at the beginning of the month, contaminating the whole 3,500 litre batch.

Whilst only valued at $2000, the costs to the farmer could be much higher because the contaminated milk was then pumped into a tanker laden with milk from other dairy farms, and he could be liable for the lot.

The last known case of deliberate contamination was in 2005, when a 30 year old Israeli was arrested and charged with contaminating 40,000 litres of milk on a mid-Canterbury farm.

The charge carries a penalty of up to 10 year's jail.