The Liberal leadership hopeful made the comments about Enbridge Inc. to reporters following a speech to party faithful inside a packed hotel ballroom in Richmond, B.C.
While he didn't address resource issues in his speech, Trudeau did discuss them earlier in the day in Calgary, saying it was wrong in the past, wrong in the present and will be wrong in the future to use resources to divide Canadians.
The issue has split B.C. and Alberta's leaders, with Premiers Christy Clark and Alison Redford acknowledging they exchanged some "frosty" words during a discussion earlier this week.
"I don't think the Enbridge pipeline is a good idea," said Trudeau after his Richmond speech.
"I believe in the precautionary principle. I believe that if you are going to build a project that goes through one of the most vulnerable and beautiful ecosystems in the world and the Great Bear Rainforest ... you have to have a better plan than what it looks like Enbridge is putting forward."
Referring to the public hearings currently taking place, Trudeau said a process is in underway to review the project, and Enbridge will have to do a better job of justifying it if it wants the pipeline to proceed.
"My default position is pipeline, sure, but not there," he said.
Trudeau also said that as a Quebec MP, he is highly sensitive to provincial areas of jurisdiction.
He said Canadians should work together and the federal government should "play a role as convener" to encourage the various regions to work out their differences.
Clark's Liberal government has set five criteria that the pipeline must meet before the province will allow construction.
The demands cover aboriginal consultations, environmental issues and B.C. receiving a "fair share" of the pipeline's economic benefits.
The pipeline is projected to carry about 525,000 barrels of bitumen a day to the B.C. coast, where it would be loaded on tankers and exported to Asia.
Trudeau spoke Thursday evening in Mississauga, Ont. before a crowd of several hundred people.