According to Reuters, this is the second biggest M&A deal with a South Korean company this year, following the purchase of the Bobcat machinery business by Doosan Infracore.
A source told Reuters: "The deal was done at 625 billion won, Macquarie is now negotiating the rest of the firm."
Macquarie bought TBC, another cable operator for a price of $879 million last year.
MBK private equity fund had previously made a bid for the whole company and will now look to join forces with Macquarie to buy the remaining 70 per cent of the company.
C&M has two million subscribers and it is the largest cable TV operator in the greater Seoul metropolitan area.
Cable operators are seen as good assets due to their monopoly in their business areas and the fact that digital TV services are likely to generate further earnings from television customers.