Birth Certificates - Carl Olaf Olsen

I would like to obtain a copy of my great grandfather's (Carl Olaf Olsen) birth certificate. The main reason is so I can find out who his birth father was. He took on his stepfathers surname, Steffensen in Australia before moving to New Zealand. His mother is Caroline Eliza Carlsen. He was born in Oslo in 1877. Unfortunately have not been able to find out what his exact birth date is but could estimate down to a month. Who should I contact? What is the normal cost for obtaining a birth certificate?