Prime Minister Justin Trudeau says he is counting on the Ethics Commissioner to get to the bottom of the SNC-Lavalin affair, but opposition politicians argue that the watchdog does not have the authority to examine some of the most important questions in the matter.

Mr. Trudeau said at the Canadian Space Agency on Thursday that Canadians can put their trust in an investigation by Ethics Commissioner Mario Dion into allegations that senior government officials put pressure on Jody Wilson-Raybould when she was attorney-general to order an out-of-court settlement of corruption and fraud charges against the engineering firm.

“Canadians need to know we have an officer of Parliament who is tasked with a specific role to make sure that, in questions where there are disagreements among politicians, among elected officials, there is an arbiter who is empowered to be like a judge, who is an officer of Parliament, who will make a determination in this issue,” Mr. Trudeau said.

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Ms. Wilson-Raybould, who was justice minister and attorney-general until January, told the Commons justice committee on Wednesday that the Prime Minister’s Office and other senior government officials repeatedly tried to persuade her to intervene in the matter.

On Feb. 11, Mr. Dion announced an investigation under the section of the Conflict of Interest Act, which prohibits a public-office holder from “seeking to influence a decision of another person so as to improperly further another person’s private interests.”

Opposition MPs and one expert in parliamentary proceedings said the real issue is not whether government officials were in a conflict of interest, but whether inappropriate pressure was put on Ms. Wilson-Raybould to order a settlement.

“This is Justin Trudeau hiding behind someone who doesn’t have the mandate to look into what he is being accused of,” Conservative Leader Andrew Scheer said.

Asked whether he would call a judicial inquiry if his party forms the next government, Mr. Scheer said that would be an option. For now, the Conservatives want the RCMP to investigate and the justice committee to call additional witnesses.

“The Ethics Commissioner conducts its investigations behind closed doors and not in public view, and there are very few consequences,” Mr. Scheer said. “This situation goes far beyond the scope of what the Ethics Commissioner is normally asked to look at.”

The NDP had hoped the Ethics Commissioner would investigate whether the PMO or someone else offered “preferential treatment” to SNC-Lavalin as it is defined in the act, rather than looking at the potential use of “influence” in favour of someone’s interests.

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“I’m concerned the Clerk of the Privy Council and the Prime Minister have been using the Ethics Commissioner’s investigation to say this is all that is needed to get to the bottom of it,” NDP MP Charlie Angus said. “He has a very limited role and he cannot go beyond that role.”

Rob Walsh, a former law clerk of the House, said it is not common for the Ethics Commissioner to delve deeply into a government decision and rule on the appropriateness of interactions between officials. The law clerk provides legal advice to MPs and committees of the House.

“I think the Ethics Commissioner is appropriately looking at whether some private interests of the Prime Minister, including partisan interests, were governing his actions, as opposed to the public interest," he said. "That is where he might find a conflict. But it’s not for the Ethics Commissioner to decide whether the public interest warranted interference by the Prime Minister. That is the Prime Minister’s call,” he said.