If there is a Delegate or Health and Safety Rep who works on your site, you should let them know first. If you're a union member and there's nobody in your workplace that you feel comfortable reporting it to, you can contact the Union office on 02 6267 1599 and we can take a safety report - it can be anonymous if you want it to. Worksafe are the Government regulator of OHS in the ACT, so you can also make an anonymous report to them on 02 6207 3000 or by email to worksafe@act.gov.au.
The CFMEU inclement weather policy states that workers will stop work and leave site when the temperature reaches 37 degrees, and lays out procedures to limit work from around 35 degrees. The temperature is measured at the nearest Bureau of Meteorology weather station to the work site
You can read a more detailed explanation and find a printable version of this heat policy here.