Government-owned retail provider Synergy will front the electricity review board and could face nearly $1.3 billion in fines after allegedly overstating the costs of generating electricity in 2016 and 2017, leading to an extra $40 million to $102 million in revenue.

As a result of a two-year investigation into nearly 15,000 wholesale electricity market trades by Synergy over 15 months between 2016 and 2017, market watchdog the Economic Regulation Authority on Wednesday alleged the generator overstated the cost to produce the energy in 12,908 of those cases.

The ERA has finished a two-year investigation into Synergy.

ERA chair Nicola Cusworth said the investigation was the biggest the authority had done in this space.

"Our key concern is to ensure that players in the market obey the rules and the rules are clear," she said.