Shameless plug: This is the best place in New Zealand to buy food, bedding and accessories for your pet rats – The New Zealand Rat Rescue store
The Lab Block they sell is a brilliant base for a pet rat’s diet, my rats just love them. Far better and cheaper than the nasty “Rat and Mouse Foods” that pet shops sell (rats and mice have quite different dietary requirements). I highly recommend lab block. You still need to give your rat fresh fruit and veges as part of a healthy diet.