After extensive searching of the World Wide Web and emailing several libraries, government departments and other people, it appears that the Monaro word is derived from an Aboriginal word. However, I am yet to receive a definite meaning for the word and from which Aboriginal Group's language the word comes from.

I have located or received the following information (as can be seen some of it conflicts):

• General Motors Holdens named the two door HK coupe 'Monaro' after the Monaro District
    of New South Wales.

• In New South Wales there is a Monaro District, Monaro Range, Monaro Plain, Monaro
    Highway, Monaro Street in the city of Queanbeyan (located on the Monaro Highway) and the
    Cooma/Monaro Shire.

• The Aboriginal people who originally lived in the Cooma area were the Ngarigo people.

• The language for the people living on the Monaro (I assume Monaro Plain) was that of the
    Ngarrugu.  The Ngarrugu people occupied the country from Queanbeyan, via Cooma and
    Bombala to Delegate. Adjoining their land the Ngunawal who occupied land around Yass,
    Booroowa and Goulburn. To the east the Bidwal.

• The Yuin-Monaro Aboriginal people originally lived around the far south coast of
    New South Wales.

• The Monaro peoples' leader's name was recorded in c1820s as Maneroo.

• The treeless plains of the Monaro are said to have been formed due to cold air drainage into the
    plains.

• The Monaro word comes from a word being 'maneroo', which means a woman's breasts or
    possibly from the same word, meaning a plain.

• The Monaro word means a 'high plateau' or a 'high place'.

• The Monaro word means 'treeless'.

• The Monaro word means 'a grassy high plain'.

• A number of different spellings are offered for monaro, including the enduring
    maneroo, and also monaroo, monera, maneiro and meneru.

• The word Monaro is a corruption of the Northern Territory Aboriginal word Maneroo,
    meaning 'breasts' or 'navel'.

• Monaro is an Aboriginal word of the people from the Monaro meaning 'a woman's breasts', or
    'paps'. (A pap is a nipple or teat.)

• The Monaro region, an Aboriginal word meaning 'breast-shaped hills'.