Samoan captain Ivan Fepuleai said their set piece play let them down.
"If we fixed up our set pieces, the game would be a lot more free-flowing," Fepuleai said.
"I think when we had the ball in hand we were a lot more deadly but full credit to Australia, they really brought it to us."
RNZ reports the next round of matches will be on Tuesday.
Samoa will play New Zealand and Australia will meet Fiji.
The tournament finishes next Saturday.
