But the company's revenue rose by 5% to $2.4 billion, underpinned by mobile services growth.
The company reported EBITDA of $664 million for the quarter, a drop of 8.7% from the $727 million for the corresponding quarter a year ago. This was put down to lower payments from NBN Co for customers who migrated to the broadband network.
For the nine months to 31 December, profit was slightly better, falling 25.5% to $431 million, compared to the $578 million of the corresponding quarter in the previous financial year.
The company's NBN customer base grew by 124,000, taking it to a total of 540,000.
Optus chief executive Allen Lew said, “We are proud to be first in market with a 5G home broadband service. Our customers can continue to expect more innovation, entertainment services, value added solutions including cyber security and game changing customer experiences in the future.
"Optus is pushing the boundaries of traditional telecommunications, enabling the company to overtake competition to become Australia’s strongest telecommunications brand, and fourth strongest brand overall."