A2 Milk rose 7.2 per cent to $20.32.
The New Zealand share market posted its biggest gain since March, boosted by gains by a2 Milk and Fisher & Paykel Healthcare.
The S&P/NZX 50 Index jumped 380.46 points, or 3.5 per cent, to 11,334.31. Within the index, 36 stocks rose, seven fell, and seven were unchanged. Turnover was $157.8 million.
The benchmark had an unexpected surge today as investors in Australia rushed to buy Kiwi milk producer a2 Milk ahead of its inclusion in the ASX 50 index. The move, announced on Friday, will see a2 take the place of embattled financial services company AMP Limited.
Adding fuel to the fire, analysts from UBS Evidence Lab yesterday upgraded A2's target price to $22.00 per share. The stock rose 7.2 per cent to $20.32.
Chief market strategist at CMC Markets Michael McCarthy said investors benchmarked to the ASX 50 may be taking advantage of a small buying window before the stock was added to the index.
AMP shares falling, despite it being a positive day for the market, added weight to the theory, McCarthy said.
The S&P/ASX 200 was up 0.4 per cent at 5pm in Wellington, but other Asian stocks were trading marginally lower.
Wall Street climbed following strong retail data. The US Dow Jones Industrial Average rose 2 per cent, the S&P 500 gained 1.9 per cent, and the Nasdaq jumped 1.8 per cent.
However, US futures are suggesting the market will be more subdued tonight.
Meanwhile, the New Zealand market had its biggest single-day gain since March 25 as the two heavyweight stocks propelled the index higher.
Along with A2, Fisher & Paykel Healthcare rose 6.2 per cent to $29.80. The healthcare manufacturer received a broker upgrade from Macquarie this morning to "outperform".
Investor sentiment was running high with broad gains across the NZX50, but companies with a large market capitalisation performed particularly well.
"If you were to strip out A2 and Fisher and Paykel then we'd probably only be up 1.5 per cent today," Grant Davies, an investment adviser at Hamilton Hindin Greene said.
"It is the big stocks that are moving upwards today and dragging our index along with it."
Ryman Healthcare increased 6.1 per cent to $13.58, Spark New Zealand rose 5.7 per cent to $4.58, Mercury NZ was up 4 per cent to $4.89 and Mainfreight climbed 2.9 per cent to $41.12.
Auckland International Airport rose 3.9 per cent to $6.75 and Air New Zealand gained 1.5 per cent to $1.655.
Tourism Holdings posted the day's biggest fall, dropping 2.7 per cent to $2.14.
SkyCity Entertainment Group was placed on a trading halt at $2.67 as the group announced it will raise $230 million in fresh equity to strengthen its balance sheet. New shares will be offered at a 6.4 per cent discount at $2.50.
Outside the benchmark index, Abano Healthcare shares fell 10.3 per cent to $2.96 after the dental clinic operator said it is considering raising between $50 million and $70 million, or taking on a cornerstone shareholder, to improve its capital structure.
The company confirmed it has received offers for the business priced at $3 and $3.25 a share from private equity firm BGH Capital and the Ontario Teachers' Pension Plan, but indicated the bid was too conditional to be attractive.
Pacific Edge surged 79 per cent to 23.5 cents after the bladder cancer specialist said one of the largest non-profit healthcare providers in the US has accepted its Cxbladder tests for use by its urologists. Kaiser Permanente has more than 12 million members and its acceptance of the testing regime is an important breakthrough for the company's entry to the US market.
Newly listed healthcare company Me Today gained 4c to 15c after signing All Black and Auckland Blues fullback Beauden Barrett as a brand ambassador.