Australian ships and aircraft are in the South China Sea as part of "normal activities", says a senior military officer.

The comment came as Foreign Minister Julie Bishop told the Lowy Institute in Sydney that Australia would object if China made a unilateral claim of an air defence zone over the sea.

Sixty per cent of Australia's exports and 40 per cent of imports pass through the South China Sea.

But there are concerns that China is putting heavy weapons onto contested man-made islands before a declaration of an air defence zone.

Australian diplomats are seeking Chinese involvement in a code of conduct covering the region.

This would avoid action that "could raise tensions, could risk miscalculation or misjudgment and could ultimately end in some form of conflict", Ms Bishop said on Thursday.

Chinese state media have been carrying columns in recent weeks threatening Australian aircraft visiting the region.

Chief of joint operations, Vice Admiral David Johnston, said on Thursday that Australia had ships and maritime patrol aircraft in the area now.

"We do routinely conduct operations through the South China Sea and have done for many years," he told reporters in Canberra.

The Australian Defence Force recently concluded a 10-day exercise, which included exercises in the South China Sea, to strengthen ties between Malaysia, Singapore, New Zealand and the UK.

Exercise Bersama Shield was based on the scenario of defending Malaysia and Singapore.

Australia's contribution included the submarine HMAS Rankin, the frigate HMAS Perth and RAAF aircraft.

"It's important we contribute to stability in that part of the world," Vice Admiral Johnston said.