It said the piece found in December by 18-year-old South African Liam Lötter was a part of a Boeing 777 "flap track fairing segment" — smooth pods under the wings housing the mechanisms that control wing flaps.
Investigators found that "all measurable dimensions, materials, construction and other identifiable features" were consistent with a 777, according to the ATSB.
A visible stencil on the part showing the serial number 676EB also matched the aircraft. Although the font and color was not the original from manufacture, the bureau added, they matched the design used by Malaysia Airlines for repainting.
The second part, found in February by American lawyer and adventurer Blaine Alan Gibson, also matched all measurable dimensions and materials for a 777 tailplane, also known as a horizontal stabilizer, which is found on the tail sections of airliners.
Like the serial number on the first part of debris, a stencil reading "NO STEP" matched the design used by the airline.
Both parts were "almost certainly" from MH370, the ATSB reiterated on Tuesday.