Which Oz region is the trickiest forecast

They're all pretty tricky, depending on the weather system. Complex coastlines like the SA gulfs can be pretty tricky. The East Coast under south swells is difficult because of the large range in wave heights (some beaches can be flat whilst others are 6ft), and Victorian beaches under westerly swells can throw plenty of curveballs too - seen plenty of swells where it's 6-8ft on the Mornington Peninsula and barely a foot in Torquay.

But, even those regions deemed somewhat 'easier' to forecast (ie SE Qld under a sustained trade flow) can be difficult at times, as you have to pay close attention to small embedded features that might not be picked up by the weather forecast models - they can often contribute a half day of bigger surf that could be the best waves of the swell event - and you don't want to miss that in your forecast notes.