"It would not be safe to put the towed sonar equipment into the water if we didn't have this kind of information about the seabed," he said.
Cost of search to be shared
Truss spoke after meeting with Malaysian Transport Minister Liow Tiong Lai and Chinese Vice Transport Minister He Jianzhong.
Liow said at the news conference that Malaysia's financial contributions to the search will match Australia's commitment.
Australia has estimated a yearlong underwater search will cost $48 million.
The Australian Transport Safety Bureau is overseeing the underwater search at Malaysia's request.
The majority of the people on board MH370 were Chinese.
All countries involved remain "cautiously optimistic" that the wreckage of the missing plane will be found, Truss said.