Top Paddock 

As a farmer it natural that you keep your best cows in the Top Paddock. If a girl is classed as Top Paddock material she is a big heifer, has a solid rump, and big thighs. Great qualities for your next BBQ but not as your guest!

Mate, shes in the top paddock, I wouldn't take her on a date anywhere.

She ain't no unicorn, shes in the top paddock for sure!

Top Paddock by Piano Player July 26, 2018

to have a roo loose in the top paddock 

An Australian colloquialism; describing someone intellectually impaired or moronic.
In the case of this phrase, the term "roo" is an abbreviation for kangaroo, an Australian marsupial.

Used emphatically, the phrase is sometimes amended to "a few roos loose in the top paddock".

To have a roo loose in the top paddock:

After John spent all those years on weed, he talked liked he had a roo loose in the top paddock.

to have a roo loose in the top paddock by MrKapper July 10, 2006

Kangaroos loose in the top paddock 

Intellectually inadequate

he's got kangaroos loose in the top paddock

Kangaroos loose in the top paddock by aenema August 16, 2003