Any income you get from your investments in the form of savings interest, dividends (even if it’s reinvested as part of dividend reinvestment plan), rental income, capital gains (from property, shares, crypto), etc. is taxed at your marginal tax rate. Investment income must be included in your tax return. Costs involved with buying, holding and selling an investment asset is tax deductible. Even the interest accrued on a loan you take to invest in something is also tax deductible. If you pay management fees to a mutual fund or pay for investment advice, those are also tax deductible.
A tax-effective investment is one where the tax you pay on the investment income is less than your marginal tax rate. Salary-sacrificed super contributions are considered tax-effective since it gets taxed at a low 15%, up to a point (source).