In Australia, SMS messages are technically 160 characters long. By default, the messages are encoded in GSM-7, which is a 7 bit encoding that only supports English alphabet, numbers and punctuation. If the non-GSM-7 characters are used, such as emoji or Chinese characters, the length of the SMS goes down to 70 characters. This is because a different encoding method has to be used in order to send those messages.
Most modern phones have support for much longer SMS messages. They can send and receive these messages as one message because they are able to join multiple SMS messages into one. But the messages are still sent in segments.
Messages longer than 160 characters are split into segments. Each segment is 153 characters long. The remaining 7 characters are used for the information about how to join the multiple messages, which the receiving phone is going to use to join the multiple messages. So, if you send a 161 character long message you will be charged for 2 segments. In this case 2 SMS messages will be sent and joined into a single SMS by the recipient's phone.
Since emojis aren't contained inside GSM-7, a different encoding system has to be use to send these messages. This system limits the segment length to 70 characters.