The New Zealand dollar held above 75 US cents as the greenback rallied after China boosted the level of cash that six of its biggest banks need to keep on hand, while the Federal Reserve's vice chair talked about the dangers associated with more asset purchases.

The Dollar Index, a measure of the greenback against a basket of currencies, rose 0.4 per cent to 77.44 after Reuters reported the People's Bank of China raised the required reserve ratio half a percentage point to 17.5 per cent, its first such move since May.

That came before Fed vice chair Janet Yellen said "accommodative monetary policy could provide tinder for a build-up of leverage and excessive risk-taking in the financial system."

She's the fourth Fed official to talk down the prospect of more quantitative easing, and investors will be looking to the minutes from the Fed's last meeting for any hint as to whether the central bank will ramp up its asset purchase programme.

"After Yellen's words of caution, I don't think QE's a guaranteed thing - the Fed minutes are the key, but the non-farm payrolls decline were all government jobs, the private sector added jobs," said Tim Kelleher, vice president of institutional banking and markets at Commonwealth Bank of Australia.

"The (New Zealand) currency didn't go up for a change after the US dollar had a positive night" and investors took some profit while Japan and the US were on holiday yesterday, he said.

The kiwi was little changed at 75.31 US cents from 75.32 cents yesterday, and rose to 66.95 on the trade-weighted index of major trading partners' currencies from 66.86. It was little changed at 61.77 yen from 61.75 yen yesterday, and slipped to 76.38 Australian cents from 76.43 cents. It gained to 54.23 euro cents from 53.95 cents yesterday and increased to 47.41 pence from 47.29 pence.

Kelleher said the currency may trade between 74.90 US cents and 75.40 cents today as it waits on the release of the Fed minutes.

Japan's yen strengthened to a new 15-year high 81.42 per US dollar while its markets were on holiday, and Kelleher said investors can expect to see strong rhetoric coming from Japanese policy makers trying to jawbone the currency. The Bank of Japan last intervened when the yen was at 83.35. It recently traded at 82.10.

Yesterday, Prime Minister John Key said the strength of the kiwi was becoming a worry, and was at a level "where it's starting to create concerns for some of our exporters." Key said China and the US need to find common ground over the where their relative currencies should be, rather than join the growing number of nations pushing for a weaker currency.

Before entering politics, Key was a member of the New York Federal Reserve Bank's foreign exchange committee.