The email addresses that are given on their individual homepages at aph.gov.au do appear to follow a standard format of senator.<surname>.aph.gov.au
A surprising number of Senators 'prefer to use the online contact form' than list their email addresses, as follows:
Mark Abib Chris Back, Cori Bernardi, Catryna Bilyk, Simon Birmingham, Sue Boyce, David Bushby, Doug Cameron, Michaelia Cash, Mathias Cormann, Trish Crossin, Christopher Evans, Don Farrell, David Feeney, Mitch Fifield, Mary Jo Fisher, Michael Forshaw, Mark Furner, Sarah Hanson-Young, Bill Heffernan, David Johnston, Helen Kroger, Joe Ludwig, Ian Macdonald,Marise Payne, Louise Pratt, Michael Ronaldson, Scott Ryan,
Julian McGauran, Nick Minchin, Claire Moore, Fiona Nash, Stephen Parry, John Williams, Penny Wong
This is not to say that they don't have email addresses that conform to the same format as those given.