Smiles could quickly be turning to frowns for some smartphone users bombarding their friends with emotion-laden messages.
A glitch means some phones are automatically turning text messages that contain emoticons, or emojis, into a pricier multimedia text message (MMS).
Unless users have a specified number of free MMS messages in their account plans, the networks will charge 50c a message.
Affected users who send four messages with an emoji a week would add more than $100 a year to their phone bill.
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Spark spokeswoman Vicky Gray said some older model phones were affected.
Samsung was aware of the issue in specific devices operating a version of Android Kitkat, including its Galaxy S4, S4 Mini and Galaxy Note II. It has developed a fix for the S4 and S4 Mini but not for the Galaxy Note II.
Samsung said users should receive a notification on their phones when a message had converted to an MMS.
Spark was speaking to other phone manufacturers to check whether any other models were also affected.
Technology commentator Peter Griffin said emojis had been a standard part of text conversations for several years.
"If you are on prepay it would rack up pretty quickly."