The competition regulator will investigate whether last year's merger between online food ordering services Menulog and Eat Now led to a substantial reduction in competition in the local online food ordering market.

Months after that deal was completed, British-listed Just Eat purchased Menulog for $855 million.

Restaurant owners fear commissions could rise due to the high market concentration. Credit:Eddie Jim

Restaurant owners have raised concerns that Just Eat controls an estimated 90 per cent of the market. The fear is that could eventually lead to a sharp rise in the commissions that restaurants pay for each order placed with the websites.

The Australian Competition and Consumer Commission never reviewed the merger between Menulog and Eat Now because no filing was made. However, Just East last year told investors the absence of a filing meant the ACCC reserved the right to assess the deal for up to six years from its completion and could impose restrictions or even require asset sales.