Buses replaced trains between Tallawong and Castle Hill on Friday after there was a power failure between Rouse Hill and Hills Showground stations after 5.20am.

Power was restored a short time later and trains began running more frequently but the system experienced a second glitch before 8am when passengers couldn't get on or off a train at Chatswood.

A Sydney Metro spokesman said the system recovered quickly after the "technical issues" were resolved, and the scheduled 68 services ran between 6.25am and 9am.

"Teething problems are to be expected on a new major railway and Sydney Metro apologises to customers," the spokesman said.

Later on Friday, a fire alarm at Macquarie Park Station compounded the network's problems.

The station was evacuated with buses temporarily replacing trains between Macquarie Uni, Macquarie Park and North Ryde.

The Metro has experienced a series of break downs during the first week of operation.

A driverless train which stopped on Wednesday had to be moved to the next station by a so-called "customer journey coordinator".

Transport for NSW blamed that disruption on a communication fault on the train between Epping and Cherrybrook stations as it was heading towards Tallawong.

The rail workers' union said the system was proving to be the debacle their members had predicted.

"(It) has failed to prove it is capable of doing the two basic tasks required of a driverless train - driving and stopping," Rail, Tram and Bus Union NSW secretary Alex Claassens said in a statement.

"We've seen commuters forced onto replacement buses, people left stranded in between stations with no information, delays and serious automatic door problems.

"We're lucky we haven't seen any major safety incidents as yet."

Almost 440,000 people have used Sydney Metro since it opened on Sunday.

Australian Associated Press