The telco blamed the drop in profit on lower NBN migration payments and workforce restructuring charges.
For the second quarter, Optus' net profit showed an even bigger fall, going down from $165 million to $105 million, a drop of 36.6%.
The company said its operating revenue increased by 6% to $4.3 billion for the half, with investment in its network and exclusive entertainment content delivering more mobile subscribers.
But pre-paid handset customers fell by 120,000 due to seasonality and the level of competition.
Optus chief executive Allen Lew said: “Optus continues to achieve profitable customer growth in the quarter and attract customers by leveraging our premium national network.
"After successfully concluding live 5G trials in Sydney we are on track to commercially launch fixed wireless access services in January 2019.
“Customer experience remains our top priority and we are focused on removing customer pain-points across the entire business."
Chart: courtesy Optus