Glanbia, which specialises in dairy processing and ingredients, said regular checks on milk supply patterns alerted it to a problem with milk from a very small number of suppliers in the Waterford area.
"This investigation centres on a possible situation whereby water may have been added to some milk prior to collection from farm," the firm said in a statement.
"We can not comment on the individuals involved other than to say that stringent penalties and actions will be taken if this is proven."
A spokesperson for Glanbia said the problem appeared to be an isolated incident and that, despite the suspected tampering, there was "no risk to the quality of the product".
It is thought a driver from the haulage company used by Glanbia was also being questioned by the firm.
Glanbia saw sales rise four per cent to €1.83bn last year, although reported a seven per cent dip in operating profit due to an increasingly competitive dairy export market and European Commission price and support cuts.
The cost pressures on the group have meant that, like several other dairy processors across the EU, it has had to contend with producers unhappy about the prices they were paid for their milk.
Richard Kennedy, chairman of the National Dairy Committee at the Irish Farmers' Association, said recently a 70-cow herd producing 25 litres of milk per cow per day stood to lose €693 in June if supplying Glanbia. He said that compared to €388.50 for Dairygold suppliers, compared to €346.50 for Kerry.