Search and rescue specialists entered the leaning Grand Chancellor hotel in Christchurch city centre on Tuesday evening.
The 28-storey hotel has been leaning to one side since the quake on 22 February.
In the past two days, thousands of tonnes of concrete have been poured into the high-rise building in an attempt to stabilise it.
There are still concerns about its integrity, but a search of the building is now possible.
So far five floors have been checked, with the stairs above that unusable.
Search and rescue staff will continue to work overnight, with sniffer dogs to be brought in to help.
Engineer David Hopkins says the concrete filled boxes placed next to the damaged wall on the building's ground floor. Mr Hopkins says damaged columns on the 12th floor will be strengthened with steel jackets and more concrete.
He says no movement has been detected in the building since Wednesday when it was seen to tilt about a metre within 10 minutes.
Hazel Rigler, a spokesperson for the building's owner, Grand Hotels International, says it is unclear whether the hotel, which opened in 1995, can be saved or needs to be demolished.
Ms Rigler says no decision can be made until engineers can get inside the building.
She says Grand Hotels has been doing everything it can to support its 109 staff members in Christchurch.