Dairy farmers who have sent cows away for winter grazing will need to make sure they comply with the new national livestock identification system, before they are returned home for the new milking season.
The NAIT animal identification and traceability scheme became compulsory for cattle on Sunday. Deer join the scheme in March next year.
NAIT requires the use of electronic ear tags, linked to a national data-base so individual animals can be traced.
That means people in charge of cattle need to register with the scheme, tag and register their stock, and record when animals move off-farm.
There's a three-year transition period for older stock not being moved off their home farm.
But new-born animals and cattle that are moved need to be registered and tagged right away.
NAIT chief executive Russell Burnard says that includes cows sent to other farms for winter grazing which should be tagged before they come back to the home farm.
He says NAIT and DairyNZ have been trying to get the message out to dairy farmers that have stock grazing elsewhere that they should make arrangements with the grazier before the stock has left and many have done that.
Mr Burnard says those that haven't been able to do that should tag the animals when they come home.
He says more than 30,000 cattle and deer farmers have registered for the scheme so far, representing nearly half of the herds.