Small and stunted plants

Tissue-cultured off-type

Cause: A genetic abnormality occurring in tissue-cultured plants.

Solution: Remove affected plants and set additional following suckers on nearby plants.
There are several causes of small and stunted plants but two important ones are burrowing nematode and banana weevil borer.

Burrowing nematode

Cause: Severe root damage caused by burrowing nematode affects the plant’s ability to take up water and nutrients from the soil.

Solution: Check a random sample of roots from the whole field to determine if nematodes are the problem and if a nematicide treatment is necessary.  

Banana weevil borer

Cause: Tunnelling by larvae of Cosmopolites sordidus in the corm. Sever infestation reduces plant vigour. 

Solution: Use stem baits (traps) to establish the severity of the infestation. If numbers exceed an average of 4 weevils per trap, treat with appropriate insecticide. Pheromone traps are also available.

Bunchy top

Cause: The banana bunch top virus (BBTV) which is spread on infected planting material and by the banana aphid, Pentalonia nigronervosa. BBTV is not present in tropical Australia, and movement of planting material is controlled by legislation.
Solution: There is no cure for this disease and all infected plants must be eradicated. Outbreaks must be reported to your state primary industry authority. 
More info:

Not enough water/dry conditions

Cause: Insufficient water for plant growth.

Solution: Use water scheduling techniques to supply sufficient water to meet the plant’s requirements.