The "odd practice" is known as leave loading - I believe its original function was to compensate shift workers for their loss of shift allowance when they were on leave. It's calculated as 17.5% of your salary for your leave period, so it works out at around 1 - 2% of your annual salary depending on how much leave you get.
Nowadays many companies pay it as an annual one off payment, normally around the end of the year (in my experience the first or second pay period in December). It's not as common as it used to be though.
The other "thing" we have over here - that confused me when I first moved over here - is Long Service Leave. Basically if you work for a company (the same company) for more than 10 years then you are entitled to between (generally) 8 weeks of paid leave in addition to your normal leave entitlement. Unlike leave loading, long service leave is a federal entitlement so most employees will get it. But you do have to stay with one employer for ten years or more.