I'm Perth based and $1sqm is about right, but also have a minimum prices set, usually around $110 - $120, it will cost your clients about $100 for four hours to hire one, they will have to pick it up, figure out how to use it, core their lawn and drop it off with that time not to mention many don't have tow bars. Return it late and the cost will go up to the daily hire charge. Add in wetting agent and fertilizer and charge a little extra. There is one contractor in Perth that charges $330 minimum and another .80c/sqm.
Remember 70% off your clients will probably be at work when you visit, they won't know how long it took you.