It is also the biggest win from a ticket purchased online, surpassing a Hokitika family's $15.7m in 2013.
The golden ticket was sold on the MyLotto website.
The prize jackpotted to $22m after Wednesday night.
It ranks as the seventh-biggest Lotto win in New Zealand.
According to QV data, the average house price in Ashburton is just under $340,000.
That means the lucky winner could buy about 65 properties there.
Ashburton deputy mayor Darryl Nelson said the win would have special significance for the small town. "I don't think they'd have to go to work on Monday if they didn't want to.
"There are a few fellas round Ashburton with a few bob but they certainly didn't win it from Lotto."
"Whoever it is it's life-changing for them, as long as they invest it wisely."
This year's Powerball millionaires include a Tauranga family that won $7m in February, an Ohakune winner with $6m in January, and a family from Timaru that won $7m in January.
Four other players around the country also won $200,000.
The winning tickets were sold at Sunnybrae Superette in Auckland, New World Westend in Rotorua, Thorndon New World in Wellington and Windsor Stationery & Lotto in Invercargill.
Strike Four has rolled over to Wednesday night and will be worth $200,000. The total prize pool for last night's Powerball draw was $22,439,853 shared among 35,373 winners. Although the first division winner claimed the bulk of that, 30,334 Kiwis scored $12 each for winning seventh division.
Strike will jackpot for next Wednesday night and will be worth $200,000. The largest Powerball was $33m won in West Auckland.
Lotto has revealed the "luckiest" stores in the country. Unichem Stortford Lodge Pharmacy in Hastings has sold 39 first-division winning tickets.
Lotto manager at the Hawkes Bay store, Nanette Woodham, has met a lot of winners. "There's a real buzz every time we sell a big winning ticket."
