New Zealand shares rallied across the board, with Mercury New Zealand and Kiwi Property Group leading the index while Scales Corp and Comvita dropped.

The S&P/NZX 50 Index gained 73.21 points, or 0.9 per cent, to 8,443.58. Within the index, 40 stocks rose, five were unchanged and five fell. Turnover was $151.3 million.

"The index has been trending up since the open really, it's a broad-based rally" said James Smalley, investment advisor at Hamilton Hindin Greene.

"There wasn't huge guidance overnight, certainly not from the US market, but it has been a reasonable day in Asia and Australia is up about the same amount as us. We're probably just trading in line with the regional bourses."

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Mercury New Zealand led the index higher, up 2.9 per cent to $3.19. Kiwi Property Group gained 2.6 per cent to $1.37 and Synlait Milk advanced 2.6 per cent to $10.

Sky Network Television gained 1.8 per cent to $2.28. The stock saw $6m in turnover today, making it the seventh most heavily traded stock on the benchmark index.

"It seems to be a stock that really polarises people: it's either a deep discounted cash play or a systemically challenged sunset business, and there's a few bids on each side - you can see that in the big crosses going either way. That will play out over the next couple of years," Smalley said.

Z Energy rose 1.4 per cent to $7.25.

An internal BP New Zealand email leaked to Stuff showed the local subsidiary of the global oil giant would raise petrol prices in Paraparaumu, Kapiti and Levin in the hope its competitors would match those prices, and reduce the price gap in Otaki where the company was losing volume.

At a press conference this afternoon, Prime Minister Jacinda Ardern said she wanted to hear BP's explanation but says the tactics underpin the need for greater investigative powers by the Commerce Commission.

The commission was already set to get greater investigative powers to trigger formal market studies, where it can compel companies to provide information, something Commerce Minister Kris Faafoi wants in place by the end of the year.

"Obviously investors at the moment don't seem to be too concerned with impact of any regulatory action," Smalley said. "The stock may be part of a broad-based rally today, but investors would be wanting to keep an eye on the issue."

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Scales Corp dropped 1.6 per cent to $4.43, while Comvita fell 1.3 per cent to $7.01 and Fisher & Paykel Healthcare fell 0.9 per cent to $12.75.

Fletcher Building dipped 0.2 per cent to $6.31. The company completed the institutional component of its $750m capital raise earlier this month, generating gross proceeds of $515m, and the retail component of Fletcher's capital raise opened on April 23 and closes on May 11.

"Prior to the rights issue, the stock was up on what appears to have been unfounded speculation," Smalley said.

"The capital raise would normally depress the price anyway, so the fact that it's held up and the strength of the institutional bookbuild, about 98 per cent took up their rights, bodes pretty well for how well it will trade once the rights entitlement is over. Retail investors who don't take up their entitlements are likely to get a reasonable price for them."