Most people have adequate exposure to Australian shares through their superannuation. Also, if Australian shares are doing well, our economy is probably doing well, jobs plentiful, high wages, happy good times for all.
A big consideration for ETFs that track the ASX300, like VAS, is that they are heavily weighted towards banks. If property nosedives, not only will your home value decline, your super is probably going to take a hit as well as any money you have invested in those ETFs. Property is a huge industry here, so mass job losses and unhappy times ahead. It's too many eggs in the basket.
I think to diversify effectively you should weight heavily overseas. My superannuation fund's high growth option only allocates 30% in overseas shares. Let's say my superannuation accounts for 20% of my net wealth. Then my overseas exposure is only 20% * 30% = 6%. IMO, no where near enough given my opinion of Australia's future.
Weight according to your portfolio as a whole. I personally like Vanguard's VGS - international minus Australia with dividends reinvested, and only 0.18% in fees.