Rory,
The Queensland MI service papers for Chalmers are in the National Archives of Australia and are available free online through them. I've just had a look at them. Here's the URL:
www.naa.gov.au/collection/using/search/index.aspx
Select "search as a guest"
New window will come up and enter "A Chalmers" in keywords and "1899" to "1902" under dates and then click "search".
New window will open and your man is the second of the three records that come up. Click the "papers" graphic icon on the far right of his line next to 1901-1902 and a new window showing the two pages of his attestation papers will open up.
I made copies so if you have any trouble send me a private message with your e-mail address and I'll send them to you.
Cheers,
Mark