This is a matter of debate, with no hard and fast answer - it comes down to many factors and personal preference. I think Vanguard goes about 50/50 in their mixed managed funds. But that's quite a high weighting to Australia from an index perspective, as Australia is only 2-3% of the world market. They cite the benefit of franking to be in favour of a higher weighting to Australia, up until about the 50% mark. But unless you're a fully committed Boglehead, I don't think everyone needs that much. Certainly from what I've read, the figure you mentioned of 25-30% would be plenty for diversification purposes.