[12][13] In June 2023, McKenzie and colleague Chris Masters won what was dubbed the ‘defamation trial of the century’,[14] a historic federal court lawsuit brought by decorated soldier Ben Roberts-Smith.

[18] He firstly worked as a cadet journalist at the Australian Broadcasting Corporation, later joining Fairfax Media (publisher of The Age and The Sydney Morning Herald).

[19] In 2009, a report by McKenzie and colleague Richard Baker into foreign bribery involving Reserve Bank of Australia subsidiaries sparked a national scandal.

[23] The reporting led to reforms of the Australian customs service announced in 2013 by Home Affairs Minister Jason Clare and overseen by former NSW judge James Wood.

[26] A 2012 interview McKenzie conducted with sports scientist Steven Dank was used by Australia's anti-doping agency ASADA in its controversial doping case against the Essendon Football Club.

[36][37] His 2017 Four Corners documentary program Power and Influence reported that ASIO had warned Australian political parties about receiving donations from two men, billionaires Huang Xiangmo and Chau Chak Wing.

The story was a catalyst for Australia's controversial counter foreign interference laws and later led to the resignation of senator Sam Dastyari over his dealings with Huang.

[44][45] The investigation also reported Australia's Department of Home Affairs favoured visa applications by Crown's VIP gamblers, including criminals.

[47] On 14 June 2020, McKenzie reporting for The Age and Nine Network's 60 Minutes Australia released covert recordings purporting to show cabinet minister and Labor party power broker Adem Somyurek organising branch stacking.

[48] Included in the numerous covert recordings, are several sections where Somyurek is heard making derogatory comments towards MPs Gabrielle Williams and Marlene Kairouz and ministerial staffers, which have been described as sexist and homophobic.

[50] In 2010, McKenzie and investigative reporters Ben Schneiders and Royce Millar revealed political parties were storing personal information about voters, raising privacy concerns.

[53] The trio's barrister said there was a public interest in whether political parties should maintain such data and that investigative journalists provide "genuine service to this community".

[56] In 2015, McKenzie defeated a Victorian Supreme Court application brought by an alleged mafia figure for disclosure of his sources in the first legal test of Victoria's journalist shield laws.

[59] The court ruled that identifying McKenzie's sources would jeopardise their safety, that there was a strong public interest in reporting on the mafia's infiltration of politics and that there would be a chilling effect if disclosure was granted.

[58] The Australian journalists’ union, the MEAA, described the decision as "important for public interest journalism,"[57] but other reporting suggested shield laws "are still far from satisfactory".

[15] The Afghanistan Human Rights and Democracy Organisation also praised the role of investigative journalism in "uncovering the truth and raising public awareness" about what had taken place in the country.