More cracks have been discovered in Sydney’s Opal Tower, meaning some residents could be locked out of their apartments for months.

On New Year’s Day, the engineering firm WSP said additional damage had been discovered on level four of the brand new building, a week after a large crack developed on level 10 on Christmas Eve.

Residents had been promised last week they could return within 10 days, after they were evacuated a second time so engineers could assess the whole building. But the wait will now be longer for those on level four.

Guy Templeton, the president of WSP in Australia & New Zealand, said the damage on level four was in the same kind of structure as the main crack on level 10, but was less severe.

“[It was] the connection between prefabricated and in situ poured concrete that was damaged on Level 10,” he said in a statement. “Two of these areas, both on level four, show evidence of some but lesser damage.”

The company that built Opal Tower, Icon Co, is installing temporary propping on the affected floor, and according to Templeton, this means some residents won’t be allowed back until further work is done.

“Units that are in the vicinity of required repairs, or are obstructed by propping, will not be able to be occupied until repairs are complete,” he said.

However, WSP said other residents could begin moving back into the tower, subject to “agreed safety processes”.

WSP, Icon Co and the developer of the building, Ecove, have all said the building is structurally sound.

On Boxing Day, the director of Ecove hit back at “sensationalist” media reporting around the tower, saying it was a “high-quality” building that only became a news story because “it happened over the Christmas break”.

Last Thursday, Icon Co’s managing director, Nick Brown, also rejected the claim that the building was a “rush job”.

The New South Wales government has announced an investigation into the tower, headed by two professors of engineering, and on Tuesday also announced a crackdown on “dodgy certifiers” in the building industry.