I'm interested in the comments that it doesn't seem like enough - I know I haven't given much details but MMM recommends it can be done on even less if you are a bit hardcore. Is it the Australian part of it that makes it seem like $850K isn't enough? Because I sure know when I look at his expenses when he has broken them down in the past I'm like man I WISH car rego was about $300 per year! A lot of stuff does seem a lot cheaper in the US although as you have said Deborah we do get other benefits around healthcare & aged pension.
IMO if you are mustachian and plan to work to earn 20-30k a year, its enough.
The nub of the issue is what are your annual expenses, both now and in FIRE? If your family can live on under 50k AUD a year then you're fine.
The math I used is:
1. old man money aka super. 200k @5% for 21 years ( hubby will work for 3 more years and stop at 39, with 21 more years to go to access super), adding 2k a year (a conservative guesstimate on super from your seasonal work) will give you 644k when hubby turns 60. At 4% this is 25k a year and at 3% its just under 20k a year. Not enough on its own.
2. 850k @4% is 34k a year and at 3% is 25k.
The super is a bit thin on its own ( you'd need old age pension to supplement, and the OAP will undoubtedly get more miserly into the future so hard to know what you can count on), so you need the 850k to keep growing. If you lived on 40k a year, earnt 30k, you'd only draw down10k and by historical measures you'd be fine. If you live on 50k, earnt 30k and drew down 20k you should still be fine. A number of Aussie families on here live well on less than 50k, myself included. However if you want to live on more than that, then the math gets a bit tighter. If I had your situation and numbers I'd aim at living off either 30/10 or 30/20, or 20/20 if you only want to earn 20k. If the market goes well and money starts piling up ridiculously then you can always change that if you really wish to succumb to consumption creep and be excommunicated from the mustachian cult.
As an additional margin of safety, you still have 1-3 years more saving and 1-3 years more super, so the math should be slightly better.
I'm not retired so I can't claim to have done this: but as Deborah says, you should make sure you can live off the "income" from your investments, which is usually regarded as the dividends paid. Fully franked dividends will also yield a tax reduction or return. By not selling any of the shares the 850k invested will grow with the capital gain of the shares. If you can organise it so that you are only drawing 10-20k net from the dividends (by earning 30k/yr), there should be some left to reinvest, yet another margin to ensure growth of your capital.
The most important factor is: what are your expenses?