What cannot be Registered and shouldn’t be used.

Marks which:

describe your goods or services or any characteristics of them, for example, marks which show the quality, quantity, purpose, value or geographical origin of your goods or services;

A simple example would be an apple farmer trying to register the phrase THE BEST APPLES. Obviously other apple farmers have a legitimate need to be able to say that they grow the best apples too. It would be unfair on other apple farmers to give one apple farmer exclusive rights to this phrase. Descriptive trade marks are difficult, and therefore expensive, to register and enforce.
Examples of some other words that are extremely difficult to register (because they are something that other traders would need to use to describe their goods or services) include:

STRAWBERRY for drinks
WARM for heaters
TOMATO for sauce
GLOBAL for freight shipping services

Geographical names on their own. Geographical references or place names are usually very difficult to register, especially when the place has a reputation for certain goods or services. For example, it would be very difficult to register TASMANIA for apples or NEWCASTLE for steel production. Traders who provide goods or services from these areas have a need to be able to indicate that place of origin of their goods or services.

have become customary in your line of trade, such as the word ‘DOG’ in relation to truck trailers or transport;

are not distinctive, i.e, a simple circle or a solely descriptive word or phrase;

are three dimensional shapes, if the shape is typical of the goods you are interested in (or part of them), or has a function or adds value to the goods;

are specially protected emblems such as Olympic emblems, National flags and emblems; Defence force emblems;

are offensive;

are against the law, for example, promoting illegal drugs; or;

are deceptive. There should be nothing in the mark which would lead the public to think that your goods and services have a quality which they do not.

Short combinations of numbers or letters on their own

Short combinations of numbers and letters (such as QL 9 or F-55 etc) are often used as serial numbers. Combinations of two letters (such as VP or HS) are commonly used as initials. Both would face significant difficulty in achieving registration. Common acronyms (such as CD-ROM or LED) or abbreviations (such as AUTO BILLPAY) are also difficult to register.

Common surnames on their own. Common surnames such as SMITH or GREEN would have problems achieving registration, as there are many Smiths and Greens in the Australian population who need to be able to use their name in relation to the goods or services they are providing. However, an unusual surname such as CREGAN or RUDNICKI has a higher likelihood of achieving registration.

And of course:

It resembles or nearly so resembles a mark already on the register.