Is this the first impression you want tourists to have of Melbourne?

An enormous betting agency advertisement showing a wallaby having sex with a lion has been painted in a field near Tullamarine Airport, designed to be visible to passengers on departing and arriving planes.

The 170-metre-by-90-metre image by online bookmaker Sportsbet, features the slogans "Rooting for the wallabies" with the twitter hashtag #rootingforoz and threatens to reignite a debate about industry standards.

It is designed to drum up business for the bookmaker ahead of the upcoming rugby union series between the Australian Wallabies and the British and Irish Lions. The first test is in Brisbane on Saturday, followed by the second test in Melbourne the following Saturday and the third test in Sydney on July 6.