These idioms are very useful in increasing your band score. Make sure you understand them properly before using them :)

IELTS Idioms for Speaking Test

To be extremely pleased or happy.

"John was over the moon when he found out he got 6.5 on his speaking test".

Something that happens very rarely. 

"China has some really beautiful and famous flowers, however, they only bloom once in a blue moon".

When something is very easy.

"Joanna studied hard for her IELTS test and got a 7 for speaking, she said it was a piece of cake".