New Zealand shares were mixed, with Synlait and A2 Milk dropping, while Mercury New Zealand and Trustpower rose.

The S&P/NZX 50 Index dipped 9.25 points, or 0.1 per cent, to 8040.42. Within the index, 23 stocks rose, 20 fell and seven were unchanged. Turnover was $137 million.

"We had a weaker lead off global markets and we're tending to trade lower," said Peter McIntyre, investment adviser at Craigs Investment Partners.

The Reserve Bank is set to release the latest monetary policy statement and official cash rate today, and "the market will be keenly waiting to see what prognosis [Grant] Spencer has for future cash rate moves - the market's not really pricing any in until 2019", McIntyre said.

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While more stocks fell than gained by the close of trading, Synlait Milk and A2 Milk Co dragged the index lower.

Synlait was the worst performer, closing down 3.9 per cent to $6.99, with A2 dropping 1.7 per cent to $7.49.

McIntyre said Synlait's association with A2 has driven the share price higher and "it's probably gotten ahead of itself", and that association hurt the stock tonight as it had dropped back due to investors lacking knowledge about A2's earnings.

"At this stage a lot of analysts are really struggling to value A2 accurately, and probably waiting to hear from the company," McIntyre said.

"A lot of the current price has already got an upgrade in it. You're going to see it stay volatile - ever since it broke through $2, it has been a volatile stock to follow.

"Its market cap at the moment is over $5 billion, its current price-to-earnings ratio is about 60 times earnings.

"It's a high beta stock, and once investors want to leave it and the buying isn't there the share price is always going to go south."

Mercury New Zealand was the best performer, up 2.1 per cent to $3.44, while Goodman Property gained 1.5 per cent to $1.32.

Trustpower rose 1.5 per cent to $5.96. On Monday, New Zealand's fifth-largest electricity generator said it had boosted first-half profit 80 per cent as its spread of generation assets allowed it to benefit from higher prices.

"A lot of their result was based on deferred costs, I'm not sure if it was actually as good as it looked but the market has continued to like it," McIntyre said.

Tourism Holdings dropped 2.2 per cent to $4.88, Comvita fell 2 per cent to $8 and Xero dropped 1.9 per cent to $34.05. Vector dipped 0.3 per cent to $3.40.

It is planning to sell 4.2 million of its own shares it currently holds as treasury stock, beginning today, as part of ongoing capital management.