Thousands of commuters face journey times of at least an extra 10 minutes when buses are used to replace train services during the seven-month shutdown of the Epping-to-Chatswood rail line from late next year.

More than 120 new buses will run as often as every six minutes during peak periods, connecting stations disrupted by the conversion of the existing 13-kilometre heavy rail line, which only opened in 2009, to carry single-deck metro trains.

The conversion of the line is part of the first stage of Sydney's $20 billion-plus metro railway from Rouse Hill in the north-west to Chatswood.

NSW Transport Minister Andrew Constance urged the more than 14,000 commuters who would be affected by the closure to "bear with us", saying they would eventually experience a "transformative" service.