Special dividend in stock market parlance generally means an...

Special dividend in stock market parlance generally means an extra payment to share holders over and above the market guidance or promised dividend payout. It is usually paid if the company's profit from trading is greater than anticipated, or company received a windfall from one-off event.
Good example is BSA, a company that has been paying for couple of years interim dividend of 1c and final dividend of one cent while earnings per share were about 4c a share. So last year they declared a special dividend of 1c, bringing the total payout for the year to 3c or 50% more than anticipated. I am a share holder in BSA and hope that they will do it again this year.
I suppose, if they did it every year, they would not call it special dividend any more.

Telstra is unique in guaranteeing their regular dividends, well almost, subject to no major unforeseen, adverse business events.
So if they decide to give shareholders more than they originally promised, they have a choice of "one of" special dividend where the money paid would be considered an income for tax purposes, or capital return, where the money would be tax free as the Tax Office would consider the sum paid as a partial return of original investment or combination of both.