Customs officials will be taking a hard line with travellers who try to cheat the new limits on duty free tobacco.

The allowance has officially been cut from today, from 200 cigarettes to 50, or 50 grams of tobacco.

Anyone bringing more over the border needs to declare it and pay duty and GST, or throw the excess away.

Officials are warning they'll seize anything which is undeclared, and anyone who deliberately ducks the rules could face prosecution.

It's part of an ongoing series of steps to reduce the harm from smoking.