Prime Minister John Key says the Labour Party should come clean on what changes it would make in the power sector if it was in government.
The Mighty River Power share offer began on Monday, with the Government intending to sell 49% of the state-owned company. It is the first of three planned for partial privatisation.
About 440,000 people pre-registered their interest and have three weeks to make a purchase. Mighty River Power is due to be listed on the New Zealand Stock Exchange on 10 May and its prospectus states the share price is likely to be between $2.35 and $2.80.
Labour Party leader David Shearer timed his warning about potential changes to electricity sector rules to co-incide with the official opening of the share flat to investors.
Mr Shearer said his party intends to reform the sector if it becomes government after the next general election in 2014.
He would not give any details, but told Radio New Zealand's Morning Report programme on Monday the changes would ease pressure on what he called increasingly unaffordable electricity prices.
However, John Key said on Monday it was a ridiculous statement to make on the eve of the share offer and if Labour is serious and wants to make radical and wholesale changes to the electricity market, then it has a responsibility to tell New Zealanders its plans.
"But if they're just politically posturing - as they appear to be to me - then I think that's a little unfortunate."
Mr Key said power prices went up 72% in the nine years Labour was in government and have gone up 10% since National took office.
Fund manager Bryan Gaynor, from Milford Asset Management, doubts many investors wanting a stake in Mighty River will be deterred by Labour's plans.
Mr Gaynor said demand among retail investors has been strong and they are unlikely to be put off.
Investors may not be deterred, but some in the industry are. A lobby group for large energy users, the Major Electricity Users Group, said David Shearer's comments add unwanted uncertainty to the future development of the electricity market.
Executive director Ralph Matthes said the market is more competitive now than a decade ago and if Labour is serious about making changes, it should focus on clamping down on rising transmission and line charges, which drive up power prices.