The trade-exposed Australian and New Zealand currencies were on the defensive on Wednesday as traders waited for the Sino-US trade deal to be signed.

The formal agreement is aimed at drawing a line under 18 months of tit-for-tat tariff hikes that have hurt global growth, although it will still not end the trade dispute between the world's two largest economy.

The Australian dollar, which is often traded as a liquid proxy for the Chinese yuan, was a shade weaker at $0.6897, treading water for a third straight day.

Its New Zealand cousin was flat at $0.6616.

"Of course, all focus tonight will be on the US-China trade deal," said Steven Dooley, APAC currency strategist for Western Union Business Solutions.

"The risk? With markets up strongly over the last three months on hopes for the deal any disappointment could see shares give back recent gains," he added.

"In this environment, the AUD might fall."

Wall Street has been upbeat since late last year on better tone on trade with the Dow up 7% while the Nasdaq has jumped 13.5% since mid-October.

US Treasury Secretary Steven Mnuchin said existing tariffs on Chinese goods would stay, pending further talks.

US President Donald Trump is slated to sign the Phase 1 trade agreement with Chinese Vice Premier Liu He at the White House at 1630 GMT.

Washington has already agreed to suspend tariffs on $160 billion of some Chinese-made electronics, and to halve existing tariffs on $120 billion of other goods to 7.5%.