"There's competition for providing that next plant," said Mackenzie, who stressed he was "not taking political sides."
"The model is used in other jurisdictions. It has its pros and cons. It's made to work."
The Labour Party's regional development spokesman Shane Jones told BusinessDesk he had taken some convincing to abandon the market model which Labour presided over during its nine years in power from 1999.
But he was convinced lower-priced electricity would create jobs in energy-intensive regional activities such as the timber industry.
The Labour-Green policy would deliver an "equity dividend" for New Zealand households and businesses by removing some of the profits currently made by power companies.
"Not only asset owners need dividends," said Jones. "Politicians need dividends as well."
To win the 2014 election, Labour needed to move about 5 to 7 per cent of the voting public to favour it.
He suggested energy analysts' capacity to "make 5 to 7 per cent of the public hate us (because of this policy) is zero. Our capacity to impress that percentage (with this policy) is infinite."
Mackenzie said the Labour-Greens policy was "focusing on small-scale generation, energy efficiency and how that works in the market not through subsidies."
He was defensive about generator-retailers trying to "pass the parcel" on rising power prices to lines companies.
Increased national grid transmission charges, reflecting a multi-billion upgrade by the state-owned monopoly grid owner, Transpower, are one reason electricity prices are forecast to keep rising in coming years. Local network charges have also been rising, although the Commerce Commission has ordered deep cuts in charges on some networks, including Vector's.
Contact Energy CEO Dennis Barnes has questioned whether the company would have invested $2.5 billion on new geothermal and gas-fired generation plant in the last five years if the Labour-Green policy had been in place.
Shares of Contact Energy have fallen about 10 per cent in the day since the policy was announced, leading to expectations the policy announcement has effectively cut the issue price for shares of state-owned generator-retailer Mighty River Power shares, 49 per cent of which are being offered to the public ahead of a sharemarket listing scheduled for May 10.
The other NZX-listed generator-retailer, TrustPower, has fallen 6.8 per cent today to break through its previous 52-week low of $7.20 to trade just after noon at $7.05. TrustPower's 50.5 per cent owner, Infratil, has fallen 2.6 per cent to $2.27. Contact, Infratil, and TrustPower are leading falling stocks in trading today