Good for you to get into the position you are in. My $0.02 on moving forwards.
As a couple of very similar age and comparable income to yourselves, and with similar goals (i.e. retire or wind down c. age 40) all I can suggest is trying to get there with residential property is completely bonkers.
Have a serious look at both commercial property and blue chip shares.
1. You won't need to run a highly leveraged portfolio with losses and be "chained" to your job. Its actually worse for you as well as it sounds like you only have one income. A resi strategy will only work if you leverage up to the eye balls, actually have some CG, then you sell down, pay tax, and re-invest in other asset classes. A mining bust, fall in resi property, or a sickness and you're completely wiped out. Especially in this market I can't see highly leveraged resi IP getting anybody anywhere fast.
2. You can easily save a deposit / have equity for a c $1m Comm IP in the near term. You have passed the "barrier to entry" that most here can't. As such stop wasting your time on resi. Such a commercial property (or shares) should yield NET $60 - 70k without a problem. Two of those and you're home and hosed.
3. You can then debt recycle from your PPOR into the Comm IP. If you are disciplined it shouldn't take long to go +cashflow on that type of investment as you pay down loans. I am not sure if you are working or not but if not think about buying in a discretionary trust.
On your rents for your resi IP's you are lucky to be cracking 3% net. You ain't gonna be retiring anytime soon on that. If you had some skill (and patience in managing a stable of slum tenancies) in acquiring high yielding resi IPs my thoughts would be different, but your are going to dig yourself a hole with your current strategy
Just to illustrate - to use your numbers:
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PPOR (soon to be IP no.3) purchased 2008 - Western Australia 3 x 1 - purchase price $490K - current value $520K - estimated rent per week $400
IP no 1 - Melbourne 2 x 1 apartment purchased 2008 - purchase price $320K- current value $410K - rent $320 per week
IP no 2 - Melbourne 2 x 1 townhouse purchased 2009 - purchase price $465K - current value $560K - rent $430 per week
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Total purchase prices - $1.46m
Total gross rent - 1150 per week, c. $60k p.a.
Estimated expenses - $3000 per property (rates, body corporate, water, insurance, etc.) = $9000 p.a.
Net Rent = $51k p.a.
Net Yield on purchase = c. 3.5%
If you had a pretty boring CIP at 6.5% yield (net) - a very conservative estimate - that would have been $95k in rent - a whole $44k better off. And if you are into industrial stuff you're yield will be a lot higher than 6.5%
As you can see you need a much smaller portfolio to get to your end destination and you would have been half way there in your accumulation phase.
And as everybody says a $2m PPOR and you'll be working til your 40's or 50's. Having a cheaper PPOR for as long as you can is key.
Anyway not advice, just my $0.02 worth.