I'm lucky I'm in a job where I don't deal with customers and 99% of the time have vague sort of deadlines such that I can disappear early one day and so long as I make up the time at some point no one minds. My employer (a large company) also has a policy that you can claim up to 10 days per year for emergency services (paid) leave which includes CFS, SES, army reserves etc.
You are supposed to get approval beforehand but for incidents I have left work for, such as the Mt Osmond fire, I went then rang my supervisor whilst on the way and said I've gone (I also told him to tell another guy at work who lives just near the golf course there that he might like to go home and protect his house!).
My point? it's up to the employer in my opinion but there are good ones around. Although it's tough if you work for a small business cos it's costing them money if you disappear.