Oct 25 (Reuters) - Shares of Australia's Magellan Financial Group tumbled more than 7% on Wednesday to hit over a decade low, after its CEO and managing director David George stepped down from his roles, with immediate effect, and as the firm flagged a rise in costs.

The Sydney-based company has been struggling through the year and had earlier flagged its eighth consecutive month of declines in assets under management to A$35.0 billion.

Executive Chairman Andrew Formica will manage operations while the company undertakes a search for a new CEO, the fund manager said in a statement.

"With the Board renewal process now complete, the business is positioned to continue the journey to restore Magellan to its place as one of Australia’s leading fund managers," Formica said.

In a separate statement, the company said it intends to make additional retention repayments of up to A$7.7 million ($4.90 million), effectively increasing its cost guidance for fiscal year 2024 Funds Management Business to between A$97.5 million and A$102.5 million.

The shares of the fund manager were down as much as 7.2% to trade at A$6.06, hitting the lowest since April 2013 and poised for their worst session since Oct. 9 as at 0040 GMT.

George, who took charge as the CEO of Magellan on July 2022, will remain until the end of the year through the transition period to a handover. ($1 = 1.5721 Australian dollars) (Reporting by Archishma Iyer in Bengaluru Editing by Shri Navaratnam)