"We have to keep training, keep fighting to get to the level we want,'' the 35-year-old said.
Added teammate Juan Martin Fernandez Lobbe: "We knew it was going to be very, very tough. We played a very bad first half. If we want to compete at this level, we have to get better at our discipline.''
Australia's Nathan Sharpe issued a warning: Argentina will get better.
"They're going to be a real strength in a year or two,'' he said. "They're going to be a real threat.''
Australia, which won three of six matches in the Rugby Championship, was just slightly better than Argentina in a match with plenty of errors and little free-flowing rugby. The loss disappointed a 40,000 sellout at Gigante de Arroyito stadium in the eastern city of Rosario.
Harris converted two of his six penalties in the first minutes one in the 3rd and another in the 8th. Argentina had little possession early with Australia dominating the play.
Juan Martin Hernandez put Argentina on the board with an 11th-minute penalty.
Harris, who is the fifth fullback used this season by Australia, converted his third penalty in the 14th to stretch the lead to 9-3. Hernandez answered with another two minutes later to make it 9-6.
Harris made it 12-6 in the 24th with his fourth penalty.
Argentina's Patricio Albacete was sent to the sin bin for 10 minutes in the 25th, reflecting the Pumas' early undisciplined play.
Harris kept up his show, converting another penalty in the 27th.
Hernandez replied in the 29th with his third penalty narrowing the margin to 15-9. But he missed two late first-half penalties that could have tied the match.
Both teams' handling was sloppy early in the second half.
Harris missed a penalty in the 57th, which would have made the lead more comfortable.
The Pumas came within inches of scoring a try in the 62nd, but they salvaged a penalty by Marcelo Bosch to make it 15-12.
Ioane broke open the game on his try in the 65th with Harris converting to make it 22-12. Harris added another penalty in the 74th.
Juan Imhoff scored a late try for Argentina in the 76th with Bosch adding the conversion for a 25-19 final.
- AP