What a lovely surprise seeing all your replies. Thanks so much for all your congratulations :)
For those who asked, I'm 47 and it's taken about 14 years.
Here's how it went, roughly (don't have the paperwork in front of me):
First year: acquired first house for $244,000
Second year: acquired second house for $135,000 and rented this one out
Third year: paying down first house
Fourth year: sold first house for $395,000; bought third house for $478,000
Fifth year: paying down third house
Sixth year: paying down third house
Seventh year: paying down house; redundancy package $65,000 which made third house mortgage free
Eighth year: sold second house for $325,000; sold third house for $508,000; purchased fourth house for $690,000 mortgage free
Ninth year: saved $15,000
Tenth year: saved $15,000
Eleventh year: saved $50,000 (total savings $80,000)
Twelfth year: saved $40,000 (total savings $120,000)
Thirteenth year: saved $30,000 (total savings $150,000); sold fourth house for $780,000; purchased fifth house for $330,000
Fourteenth year: total assets (house, shares, super): $1,003,340
Current income per year = $36,556
Current expenses per year = $27,720
The good news is I've just resigned from my job, but I'll look for another job to help pay for some improvements to my house and I also want to buy more shares or bonds, to help secure and hedge my position.
In fact, I'd like to move my focus from houses to 'proper' investing. The houses were attractive because our market has been going up over the fourteen years and we're in a boom now, so any improvements to my house will more than pay for themselves if I sell. We've got limited space and high population growth (if anyone's wondering why the market's so good!)
Doing my networth the other day was quite a surprise; it all snuck up on me while I was busy saving/buying houses/making super contributions.