Some Spark systems including the website, app and voicemail services are down.
Currently, the pages on the Spark website says "maintenance is in progress".
"We're aware of an issue currently impacting our back-end systems. This means customers may be unable to call Spark, access our website, app, voicemail services, or their Xtramail," Spark said in a statement posted on social media.
Customers' broadband, landline, and mobile services are unaffected.
Due to the systems being down, Spark said the opening of some stores may be delayed on Tuesday.
"We are working with urgency to get our back-end systems up and running and will update customers here as we know more."
