Four former Supreme Court justices were, in one way or another, caught up in the high-profile SNC-Lavalin affair — a surprising revelation stemming from Ethics Commissioner Mario Dion's report.

Frank Iacobucci, in his role as SNC-Lavalin's legal counsel, prepared a legal opinion for the government outlining why it was legitimate for then-justice minister and attorney general Jody Wilson-Raybould to intervene in criminal matters handled by the Public Prosecution Service. He sat on the high court from 1991 to 2004.

Dion's report revealed that Iacobucci also requested an opinion from John Major, who served on the bench from 1992 to 2005, on the legality of the Director of Public Prosecutions refusing to give SNC-Lavalin a deferred prosecution agreement — and not providing reasons for that decision.

Iacobucci had also reached out to former chief justice Beverley McLachlin, who served on the Supreme Court from 1989 to 2017, and she reviewed the SNC-Lavalin file. McLachlin was also approached by Prime Minister's Office officials about giving advice to Wilson-Raybould, but she declined the offer.

Thomas Cromwell, a Supreme Court justice from 2008 to 2016 who was not named in Dion's report, had been tapped by Wilson-Raybould to advise her on the limits of solicitor-client privilege after she resigned from Prime Minister Justin Trudeau's cabinet.

Wayne MacKay, professor emeritus of law at Dalhousie University, said one of the key principles when you're a judge is that you are removed from politics, and objective about the political world.

"That doesn't really apply when you cease to be a judge, but it's a bit jarring to hear about judges giving opinions on highly charged political events like this," he told CBC News.

MacKay said it's an emerging issue because judges used to be appointed at an older age, and disappeared from the public scene after they retired. Increasingly, judges are carrying on with significant careers after they leave the bench.

"While at one level they are retired and they should be free to do as they wish, to some extent they are still going to be viewed in the general public with the court. It's part of their attraction as advisers, their impressive careers with the court," he said.

MacKay said it would be a good idea to examine the issue to determine whether the trend should simply be accepted as part of the modern world, or whether ethical guidelines that apply to sitting judges should extend to retired judges.

Renée Thériault, executive legal officer for the Supreme Court of Canada, said like all Canadians, many former judges are living longer, healthier lives and choose to continue to work after retiring as justices. The question of post-judicial employment is "complex and involves many considerations," she said.

When a former judge intends to pursue a legal career, he or she must abide by rules governed by the law society in their jurisdiction. Individual courts also have rules which may bar a former judge from appearing in court or impose a cooling-off period.

"The Canadian Judicial Council and the Federation of Law Societies have been discussing ethical principles that would complement the rules. We expect that the revised Ethical Principles for Judges will put forward additional considerations in this regard," she said. "These considerations would apply to all former federally appointed judges whether they be from the Supreme Court or otherwise."

Supreme Court Chief Justice Richard Wagner, who is also chair of the Canadian Judicial Council, is leading the modernization of ethical principles for judges.

"As society evolves, so do the ethical issues that judges sometimes face," he noted when public consultations for the review process were launched in March.

A background paper says judges' activities after retirement is among the themes under consideration.

"This issue considers ethical issues regarding pre-retirement discussions and limits on post-retirement professional activities," it reads.