The user, who preferred to remain anonymous, told iTWire that he was first redirected to a site which did not load in full and displayed the name "coachtraffic.com" in the browser bar.
He was then redirected to a page which loaded fully, with the domain reading mac-safetycheck.com followed by a number of letters which did not spell out any pronounceable word.
The text on this page read: "Your system is potentially infected with 3 viruses. Our security check found potential traces of 2 malware and 1 phishing/spyware.
A button invited the user to "Scan now".
The user, who is highly computer literate, said he was running version 10.14.3 of the macOS operating system and using an anti-virus program by the name of Intego on his MacBook Pro.
Another message further down the page said: "IMMEDIATE ACTION REQUIRED We have detected a potential trojan virus on your Mac. Press OK to begin the repair process."
iTWire loaded The Guardian Australia site on Firefox and Chrome on a Debian GNU/Linux system, but there was no sign of any redirect as experienced by the user.
The Guardian has been contacted for comment.