New Zealand shares edged lower in subdued trading as fears over the country's trade relations with China unnerved investors.

Property For Industry fell after its earnings missed expectations, while Port of Tauranga hit a new record when upgrading its annual earnings guidance.

The S&P/NZX 50 index decreased 0.21 points, or 0.002 per cent, to 9,245.44. Within the index, 29 stocks fell, 16 gained, and five were unchanged.

Turnover was $89.8 million, with just five companies trading on volumes of more than a million shares. New Zealand's benchmark index was one of the few to decline across Asia, with most markets buoyed by the progress made in the US-China trade talks.

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New Zealand's own relationship with China took centre stage after reports last week that primary goods exporters were facing issues clearing customs and that tourists were being discouraged from visiting New Zealand.

Prime Minister Jacinda Ardern played down those concerns at her weekly post-Cabinet press conference, saying the two nations' relationship was significant and complex and brought great benefits to both parties.

Greg Smith, head of research at Fat Prophets, said New Zealand was more heavily reliant on China as a trading partner than the US, and as a much smaller nation couldn't throw its weight around as easily.

"We've got to tread carefully given we've got so much to lose," he said.

A key bone of contention between the nations was the Government Communications Security Bureau's rejection of Huawei Technologies' equipment for Spark New Zealand's planned 5G mobile network.

The UK government has reportedly found a way to mitigate the risk from using Huawei kit, and Spark has said it's hopeful of using its existing partner's technology.

Spark slipped 0.1 per cent to $4.06 on volume of 2.3 million shares, the most heavily traded stock on the day.

Of other stocks trading on volumes of more than one million shares, Trade Me dipped 0.2 per cent to $6.38, Goodman Property Trust slipped 0.6 per cent to $1.64, and Mercury NZ declined 0.8 per cent to $3.75. A2 Milk, which has relied on rapid growth in China to become New Zealand's biggest listed company, rose 3.6 per cent to $13.16 on a volume of 1.2 million.

That was the biggest gain on the NZX 50. Fruit exporter Scales Corp gained 2.8 per cent to $4.73 on a volume of 430,000 - its three-month average is 61,000.

Sky Network Television led the local market lower, down 3.3 per cent at $1.77 on a volume of 417,000, near its 90-day average of 477,000.

Sanford, which complained of salmon getting held up at China's border last week, fell 2.8 per cent to $6.70 on typically light volumes of 67,000. Property For Industry was down 0.8 per cent at $1.915 on slightly busier volume than usual of 280,000.

The industrial property investor reported flat underlying earnings today, missing analysts' expectations as a lower-than-usual occupancy rate and increased property costs crimped its net rental income. Other property investors were also weaker.

Kiwi Property Group fell 2.4 per cent to $1.405, Argosy Property was down 2 per cent at $1.22, Stride Property declined 1 per cent to $1.96 and Precinct Properties New Zealand decreased 0.3 per cent to $1.535.

Port of Tauranga traded at a record $5.45, ending the day up 0.4 per cent at $5.25. The port operator reported a 4 per cent increase in first-half profit and said annual earnings would be at the top end of guidance on rising cargo volumes.

That was boosted in part by congestion on rival Ports of Auckland's wharves.

Outside the benchmark index, Steel & Tube Holdings rose 0.8 per cent to $1.23 after reporting a 49 per cent increase in first-half profit, and said it would resume paying dividends with an interim payment of 3.5 cents per share.

Of companies reporting tomorrow, Heartland Group gained 2.3 per cent to $1.34 and NZME rose 6 per cent to 53 cents.