In Australia, while appreciated, tipping can be the exception rather than the norm. As a guide, tipping is appropriate though not required in a restaurant for a meal (10-15% as a general rule). In a taxi, a cafe or a bar, rounding up is acceptable.
You can consider tipping your leader for outstanding service throughout your trip. The amount is entirely a personal preference; however, AUD2-5 per person per day can be used as a guideline. Of course, you are free to tip more or less as you see fit, depending on your perception of service quality and the length of your trip. Remember, a tip is not compulsory and should only be given when you receive excellent service.