'I'
Icy pole Popsicle
Identity -old- An old, noted inhabitant of a locality
Idiot box -the- The television
Iffy -a bit- It's risky; it's suspect
Illywacker A trickster or spieler
Imbo A trickster's victim

Improve -on the-

Improving in health or proficiency

In smoke In hiding
In the bag -it's- It's as good as done

In the blue

Out of control

In the book

Being all right, but not very smart

In the wool Ready or nearly ready for shearing
Info Information
Inked Drunk

Inland

Any part of the Australian mainland that is not on the coast, specifically the outback or remote central areas of the continent

Inside country Well populated country near or in coastal areas
Inside squatter A farmer or large land owner in a fairly well populated district
Iron -an- A stirrup
Iron man A £ 1 note
Ironbark Strong; unyielding
Irrits -you give me the- You are extremely irritating
Issue -the- Everything; the lot
It's a gig It's extremely good
Ivories -your- Your teeth