Section 10 TMA provides the following definition of “deceptively similar”:
“For the purposes of this Act, a trade mark is taken to be deceptively similar to another trade mark if it so nearly resembles that other trade mark that it is likely to deceive or cause confusion.”
Windeyer J in Shell v Esso at first instance explained the approach applicable to deceptive similarity as follows:
On the question of deceptive similarity a different comparison must be made from that which is necessary when substantial identity is in question. The marks are not now to be looked at side by side … the comparison is … between, on the one hand, the impression based on recollection of the plaintiff’s mark that persons of ordinary intelligence and memory would have; and on the other hand, the impression that such persons would get from the defendant’s … exhibitions … The likelihood of deception must be judged not by the degree of similarity alone, but by the effect of that similarity in all the circumstances.
In Australian Woollen Mills Ltd v F S Walker & Co Ltd, Dixon and McTiernan JJ described the proper approach as follows:
In deciding this question, the marks ought not, of course, to be compared side by side. An attempt should be made to estimate the effect or impression produced on the mind of potential customers by the mark or device for which the protection of an injunction is sought. The impression or recollection which is carried away and retained is necessarily the basis of any mistaken belief that the challenged mark or device is the same. The effect of spoken description must be considered. If a mark is in fact or from its nature likely to be the source of some name or verbal description by which buyers will express their desire to have the goods, then similarities both of sound and of meaning may play an important part. The usual manner in which ordinary people behave must be the test of what confusion or deception may be expected. Potential buyers of goods are not to be credited with any high perception or habitual caution. On the other hand, exceptional carelessness or stupidity may be disregarded. The course of business and the way in which the particular class of goods are sold gives, it may be said, the setting, and the habits and observation of men considered in the mass affords the standard. Evidence of actual cases of deception, if forthcoming, is of great weight.
To show that a trade mark is deceptively similar to another it is necessary to show a real tangible danger of deception or confusion occurring. A mere possibility is not sufficient. 
Allowance must be made for imperfect recollection, careless pronunciation and speech. Ultimately, this issue of fact “depends on a combination of visual impression and judicial estimation of the effect likely to be produced in the course of the ordinary conduct of affairs”.
In Delegate of Trade Marks v Woolworths Ltd, French J noted that:
All surrounding circumstances have to be taken into consideration. These include the circumstances in which the marks will be used, the circumstances in which the goods or services will be bought and sold and the character of the probable acquirers of the goods and services.
The respective trade marks should be compared both visually and aurally.
An Applicant’s mark may be deceptively similar to an Opponent’s mark if the Applicant’s mark incorporates an “essential” and “distinguishing” feature of the Opponent’s mark. The identification of such an essential distinguishing feature of a mark requires the approach described by Lord Radcliffe in Saville Perfumery:
“The identification of an essential feature depends partly on the Court’s own judgment and partly on the burden of the evidence that is placed before it. A trade mark is undoubtedly a visual device; but it is well-established law that the ascertainment of an essential feature is not to be by ocular test alone. Since words can form part, or indeed the whole of a mark, it is impossible to exclude consideration of the sound or significance of those words.”
 Shell Company of Australia Ltd v Esso Standard Oil (Australia) Ltd (1963) 109 CLR 407.
 (1963) 109 CLR 407 at 415, see also CA Henschke & Co v Rosemount Estates Pty Ltd (2000) 52 IPR 42, at 61, per Ryan, Branson and Lehane JJ.
 (1937) 58 CLR 641 at 658.
 Southern Cross Refrigerating Co v Toowoomba Foundry Pty Ltd (1954) 91 CLR 592, at 594-595, per Kitto J; Delegate of Trade Marks v Woolworths Ltd (1999) 93 FCR 365, at 382, per French J (Tamberlin J agreeing).
 Australian Woollen Mills Ltd v FS Walton & Co Ltd (1937) 58 CLR 641 at 658–9 and Crazy Ron’s Communications Pty Ltd v Mobileworld Communications Pty Ltd (2004) 61 IPR 212 at –.
 (1999) 45 IPR 411 at 428.
 Shanahan 4th ed [30.1535]; Wingate Marketing Pty Ltd v Levi Strauss & Co (1994) 28 IPR 193.
 Crazy Ron’s Communications Pty Limited v Mobileworld Communications Pty Limited (2004) 209 ALR 1 at - per Moore, Sackville and Emmett JJ, Saville Perfumery Ltd v June Perfect Ltd (1941) 58 RPC 147 per Greene MR at 162.
 Saville Perfumery Ltd v June Perfect Ltd (1941) 58 RPC 147 at 105.