I don't think extending handling time is the answer if shipping internationally, as whatever you set will apply to domestic orders and ETAs as well. If Aus Post are sending some orders by sea mail, the only option would be to have a 30 day handling time (6 weeks) to counter it, which would give local buyers ETAs of around 7 weeks. 

The better option is to (if possible) select a flat rate postage option which has a longer delivery time, such as the international economy one, which allows for 35 business days (7 weeks) in addition to your handling time, without affecting local ETAs by the same amount (also, don't forget dispute timeframes are based on ETAs - personally I would not be looking to give local buyers 3 months or so to raise eBay claims).