A Vodafone spokesperson said there was a widespread outage "related to mobile data" and it was currently being investigated.
It appeared to be affecting people on accounts, rather than pre-pay, and the outages "seem to be intermittent".
"Our tech teams are urgently investigating and repairing a nationwide issue impacting some mobile data services," Vodafone said in a statement.
"Some customers are experiencing an intermittent loss of 4G internet connectivity, which we believe is impacting PostPay (on account) customers only.
"We hope to have this fixed ASAP."