Under the old CGT system, to calculate your tax, the realised gain is included in your taxable income and will be taxed at your marginal tax rate. Indexation is allowed on the cost base of the shares which were held for longer than 12 months prior to the disposal.
Indexation increases the cost base by the rate of inflation.
The Consumer Price Index table is available below for your reference. If the shares are sold 12 months or more after the date of acquisition, then only half of the realised gain from the disposal of these shares will be included in your taxable income. If the holding period was less than 12 months, no concession under the new system will apply to you and the full value of realised gain will be included in your taxable income. Under the new system, there is no indexation of cost base. On the other hand, you can retain a degree of indexation of the cost base, but forgo the discount.