Capital gains is a reasonably straightforward concept, and applies to more than just real estate. A capital gain is the difference between what you paid for something and what you sold it for, less expenses. For example if you bought a house for $100,000, spent $40,000 on renovating it and sold it for $200,000 you have made $60,000. That example is a bit simplistic as you can also subtract buying and selling expenses such as stamp duty, agent's commission, and any interest you paid while you held the house. If you are renovating or building to sell for profit, keep all your receipts and loan bank statements for your accountant.
Capital gains tax is not the same as capital gains. It is a tax you pay on any amount of capital gains that you make. Capital gains tax is calculated at your normal rate, so it is just counted as regular income and taxed no differently to income you'd get from a job. If your other income is exceptionally low or your only income is from capital gains then expect to pay very little tax on your capital gains.
There are some exceptions to capital gains tax for real estate. If you have owned a property more than a year, the amount you are taxed on is halved. In the earlier example, if you carried out the renovation in less than a year the full $60,000 would be taxed, but if you took longer than a year only $30,000 would be taxed. This is to discourage people flipping houses and give a bonus to people who have held a property for a longer time.
The other big exception that can make a huge difference if you are on a low income is that capital gains from your own home is exempt. That is, if you renovate your own house, sell it at a profit and do it again, you don't pay any tax. The exception to this is if you are doing it so frequently that you are deemed to be renovating as a business, and then all your income from capital gains is assessed.
The impact of capital gains on Government pensions is more varied. Profit from your own home is not counted as income to Centrelink, but you do need to show what you have done with the money after the sale and if you have it in an interest-bearing account then the interest will be deemed as income. If you declare capital gains from a project that is not your own home then it will probably fall under the category of 'business income' and it will be deemed to have been earnt over a fairly long period, maybe 6 months or a year and will reduce your income accordingly for some time in the future. If you are also earning interest on the money then that will be taken into account too. Of course, check with Centrelink as personal circumstances can vary.
If you are a very low income earner then the best strategy is straightforward. If you are going to build or renovate for profit with the smallest impact on your pension, do it on your own home only and do it on a very leisurely time frame, say 2-4 years per house. This kind of time frame gives you lots of time to establish the garden, should give you some time to enjoy the house after the renovations are complete and has less of an impact on children than attempting to do very frequent renovations.