The Commerce Commission has issued a formal warning to two energy companies over conduct deemed anti-competitive during the purchase of the Cobb power station near Nelson in 2002.
Contact Energy and TrustPower were investigated for bid-rigging behaviour, although a prosecution was never brought.
The commission says the companies had gone too far in discussions initially aimed at avoiding competitive bidding.
Also, they had communicated over a possible hedge arrangement for the supply of electricity from Cobb, depending on which company bought the power station.
Commission chair Mark Berry says it decided not to prosecute because a hedging arrangement was never actually made and the companies' discussions did not appear to affect their bidding behaviour.
However, Mr Berry said the companies' intent was clear.
"Parties need to remember that attempts are equally in breach of the act as if they had in fact entered into those arrangements."
TrustPower and Contact have issued brief statements, saying their actions were never anti-competitive, but they will accept the warning and move on.
TrustPower says the $92.5 million it paid for the Cobb station in 2003 was significantly more than the next tender.
Contact Energy says the discussions were pro-competitive.