But a high-level review, ordered after the system went into meltdown for two consecutive days, stopped short of recommending the new timetable introduced in late 2017 be scrapped entirely.

A report by Sydney Trains and Transport for NSW, released Thursday, acknowledged the major disruptions on January 8 and 9 were "highly unsatisfactory" and arose from a "rare combination of causes".

The city's "tangled" and "complex" rail network made it vulnerable to disruption, while underlying issues with crew availability meant not enough drivers could get to where they were needed quickly enough.

"We apologise to customers, who showed extreme patience for two consecutive days last month as they were tested by disruptions to a level rarely seen," transport secretary Rodd Staples said in a statement.

The report made eight recommendations, including that the government accelerate recruitment of new drivers.

It noted that Queensland and Victoria had ongoing campaigns to hire drivers and were offering more money.

The review also suggested "minor adjustments" to scheduled off-peak services that are used by very few people, to provide extra capacity when incidents hit the peak periods.

Simplified changeovers for crew, updated IT and communication systems and formalised protocols for incident recovery were also recommended.

"While the timetable has proven it meets reliability targets during normal operation and routine incidents, when we have significant incidents our resources become stretched too quickly," Sydney Trains chief executive Howard Collins said.

Transport Minister Andrew Constance said the government would enact all the recommendations to ensure recovery from major incidents was improved.

"We have seen unprecedented growth in public transport patronage, and it is important we respond to this - while also planning for a growing city with extra services," he said in a statement.

Opposition leader Luke Foley said the report proved the system was under-resourced and the new timetable was not working.

"The government's now going to have to reduce train services because the timetable they introduced is a mirage," he told reporters in Sydney.

"This report is the white flag on their new timetable."

The Rail, Tram and Bus Union agreed, saying the review fully vindicated workers who warned the government the timetable was unsustainable and that chronic understaffing had left the entire system vulnerable.

NSW secretary Alex Claassens said rail staff and commuters now wanted to see action.

"Let's get on with taking onboard those recommendations, taking action and fixing this rail network," he said in a statement.

Overtime, rostering and the new timetable have been key issues in a pay dispute between unions and management, which remains unresolved.

Australian Associated Press