"China's intent is crystal clear. We ignore it at our peril," he said in public testimony. "I'm concerned China will now work to undermine the international rules-based order."
Plain-spoken and well-known in the international community for his remarks on US policy in the Asia Pacific, Harris has often provoked a vitriolic reaction from Beijing, in particular for his passionate calls for action in the South China Sea.
His appointment would raise the stakes in the battle for influence in Asia, with experts saying Harris could push the Australian government to tighten military cooperation with its traditional ally.
Harris' appointment must be confirmed by the US Congress, and it's not clear exactly when that will happen.
During his testimony to the committee on Wednesday, Harris said China was trying to reorder the Indo-Pacific through "military modernization, influence operations and predatory economics."
"China's impressive military build up could soon challenge the US across almost every domain ... If the US does not keep pace, (US Pacific Command) will struggle to compete with the People's Liberation Army on future battlefields," he said.
Born in Yokosuka, Japan, the son of an American naval officer and a Japanese mother, Harris ascended through the ranks of the US Navy to become the highest commander in the Asia Pacific region.
Carl Schuster, former director of operations at the US Pacific Command's Joint Intelligence Center, told CNN that Harris was greatly respected by the armed forces and in political circles in Washington DC.
"He may be a good advocate for Australia's interests and position ... He has credibility with the Trump administration so he could very well be viewed as two-way conduit," he said.
Australian leaders warmly welcomed the news of Harris' appointment, including Prime Minister Malcolm Turnbull who has met with the admiral in Hawaii on at least one occasion.
Schuster said Adm. Harris was an "innovative and flexible thinker," who was a strong advocate for a strong advocate for tighter ties between the US and its allies in Asia.
The admiral's harsh rhetoric comes at a time when Australia finds itself caught between its longtime ally the United States and its major economic partner China.
Euan Graham, director of the International Security Program at Sydney's Lowy Institute, told CNN Harris could help the Trump administration put more pressure on Australia to follow the US line.
"Australia can't continue to expect that it will be able to have its cake and eat it too when it comes to the economic relationship with China and (its US ally),"
"(Harris) is someone who is prepared to take the gloves off when it comes to making hard choices in the relationship between China and the US."
"His nomination singles an interest on the part of the administration to make sure that Australia steps up on questions of defense cooperation, alliance cooperation, where China is concerned," he said.
Harris has been a longtime critic of the Chinese military's island building campaign in the South China Sea, a position he continued on Wednesday citing Beijing's "aggression" in the area.
Schuster said rather than having an aggressive attitude towards Beijing, Harris instead had a "realistic" approach to China.
"He sees China as a strategic competitor to the US ... he looks at the South China Sea and thinks if we keep ignoring what China is doing there, then China will win by default," he said.
The Chinese government has expressed its open dislike for Harris on multiple occasions in state media and is unlikely to be pleased at the news of his ambassadorship.
"Some may say an overemphasis on his background as a Japanese-American general is a bit unkind .. But to understand the Americans' sudden upgraded offensive in the South China Sea, it is simply impossible to ignore Admiral Harris' blood, background, political inclination and values," the piece read.
When asked about Harris' nomination earlier this week, Geng Shuang, China's Ministry of Foreign Affairs spokesman, stressed the need for stability.
"It's a shared responsibility and expectation of countries in the Asia Pacific region to promote stability and development. China hopes the policies and relations of involved countries can benefit the peace and stability, development and prosperity of the region."
Graham said any loud protests by the Chinese government could backfire, particularly given the recent atmosphere of outrage in Australia over allegations of attempts by Beijing to influence domestic policy.
"I think the response, by and large, in Australia would be to swing behind the US alliance," he said.
CNN's Serenitie Wang contributed to this report.