Lobby group Safe Kids says new requirements for children travelling in cars do not go far enough.
The Government on Thursday announced that children will have to ride in booster seats until they are seven years old, up from the previous age limit of five.
Safe Kids' director Ann Weaver says she is disappointed the changes are at odds with international best practice.
Ms Weaver says experts recommend that children use booster seats until they are 148 centimetres tall, or roughly 11 years old.
Associate Transport Minister Simon Bridges says the Government acknowledges the international evidence, but has set the age at seven because it is simplest for parents to understand.
Mr Bridges says the cost is not a major factor, as car seats can now be found for as little as $30.
A public education campaign will be held before the changes begin being enforced in about a year. After that time, any parent without a child properly restrained could be liable for a fine of $1000.