SYDNEY, March 19 (Reuters) - Australia's Macquarie Group and Canada's Public Sector Pension Investment Board are starting a sales process for their AirTrunk data centre business, according to a source with direct knowledge of the matter.

The pair sent non-disclosure agreements on Tuesday to potential buyers to sign ahead of a formal transaction process, the source said.

Australian media reported AirTrunk could be valued at up to A$15 billion ($9.77 billion) and a majority stake could be sold as part of the process.

Macquarie and Public Sector Pension Investment Board, also known as PSP Investments, together own 88% of the data centre business but have not disclosed their individual stakes. Macquarie owns its stake in AirTrunk through its Macquarie Asset Management unit.

The source could not be named discussing confidential information.

Macquarie Group declined to comment. AirTrunk did not immediately respond to a request for comment. PSP Investments did not respond to a request for comment sent outside normal business hours. ($1 = 1.5349 Australian dollars) (Reporting by Scott Murdoch; Editing by Jamie Freed)