If the chorus isn't stuck in your head, it will be soon.

Melbourne Metro Trains' darkly cute - and irksomely catchy - new ad for transport safety has gone viral, notching up a whopping 2.7 million YouTube views in just 48 hours.

And nobody is more stunned by its success than the man behind the music, Sydneysider Ollie McGill.

The Cat Empire keyboards player was commissioned to write the score to accompany lyrics to the McCann Group's new ad and has watched Facebook likes, Twitter shares and YouTube hits skyrocket as word of the animated video has spread like wildfire.

''I'm always a bit self-critical - I just didn't expect even a tenth of even a hundredth of this effect,'' he said.

''I'm kind of overwhelmed to tell you the truth. I'm not a major songwriter and I've never dreamed of something I've written going viral like this.''

In the ad, cartoon characters meet their ends in a number of colourful, sardonic ways, including a couple of nasty mishaps on train tracks, while the sweet chorus, ''dumb ways to die … '' is instant earworm material.

McGill, 31, of Wahroonga, says he is thrilled at the uptake - and sees something of an advertising revolution in the song being downloaded on iTunes.

Within 24 hours, it was in the top 10 chart of iTunes and by Sunday was ranked number six in the global iTunes chart. The song was on the front page of Reddit for six hours.

''It's gone beyond the desired effect, which was ambitious to start out with,'' the married father of one said. ''It's mad - I don't know when it will stop.''