Optus ended the December quarter around 8.9 million mobile subscribers, according to parent company SingTel (ASX:SGT).
Optus recorded net additions of around 88,000 customers during the quarter. The operator's postpaid customer base hit 4.6 million, up 14 per cent from a year ago.
But prepaid customers declined by 62,000 over the same period, with additions impacted by a higher than average churn rate.
The increased number of smartphone customers also helped earnings, the company said.
Optus reported a two per cent rise in third quarter net profit to $169 million as it says competition in the mobile sector remains high.
Operating revenue in the three months to December 31 was $2.38 billion, up 3.6 per cent from the previous corresponding period, driven by growth in its mobile division "in spite of a highly competitive market".
Mobile service revenue was up 6.8 per cent on the previous corresponding period, while the division's earnings were up 3.8 per cent to $371 million.
The third quarter result took Optus' net profit for the nine months of its fiscal year to $514 million, up 12.7 per cent on the previous corresponding period.
It maintained its full year outlook of growth in operating revenue and earnings at mid single-digit levels.
Optus is fully owned by Singapore Telecommunications Ltd (SingTel), which posted $998 million third quarter profit, up 0.8 per cent from the previous corresponding period. Group revenue was up six per cent on the previous corresponding period to $4.7 billion, driven by the performance of Optus and its Singapore operations.
SingTel said its consolidated operating revenue and operational earnings would be impacted by exchange rate movements in the Australian dollar.
Growth from SingTel's regional affiliates, including India's Bharti Airtel, helped the company grow its groupwide customer base to 383 million by the end of the year.
Locally listed SingTel shares were up one cent at $2.39 at 1021 AEDT.
With additional reporting by AAP