As someone living in Sydney this article is ignoring some realities... They may have made a profit on those flips but a generous amount of those profits were not from the work they've done but the fact that prices in Sydney have risen 30% in three years. It's also ignoring some pretty significant costs.
For example stamp duty on a $1M property would be a bit over $40K (https://www.hsbc.com.au/1/2/personal/home-loans/calculators/stamp-duty). So that's 15% of their "profit" gone. If they're using standard financing they would have to "down pay" 20% or pay for LVR insurance which can be significant. On 10% down the LVR insurance would be between $20-30K. That doesn't include lawyers, conveyancing and building inspections, which probably wouldn't add a significant amount. So that's what $70K x 3 houses that hasn't been taken into account in their $600K profit... Then they are splitting the profits between three of them.
It's still a reasonable income per partner (I'd certainly be happy to get that income) but the article is inflating the benefits and ignoring all risks.
We're at all time low interest rates at the moment. What happens to the market when all these people under mortgage stress now, can't cope when interest rates inevitably start to increase again?