The Eastern Bay Energy Trust looks set to block a $50 million partial takeover bid for NZX-listed Horizon Energy.

Marlborough Lines filed a notice with the stock exchange this morning, offering $3.96-a-share for 51% of Horizon.

The bid valued the Marlborough Electric Power Trust-owned company at $99 million. The intended partial takeover offer is subject to a minimum 51% condition and starts on September 29.

But the Eastern Bay Energy Trust is a 77% shareholder of Horizon Energy and the trust chairman, Kevin Hennessy, said it was "definitely not" a willing seller.

He said the board still had to discuss it, but the trust generally favoured local community ownership. "We looked at merging with lines companies last year and decided we're better off staying on our own."

Whakatane-based Horizon Energy runs power lines servicing the eastern Bay of Plenty. A statement this morning said Horizon's board of directors would consider the offer and appoint an independent advisor.

Marlborough Lines chairman David Dew said there was a process to go through before the bid was successful, starting with a valuation. "It's a drawn-out process and hard to predict but we think the proposal has a lot to offer."

Mr Dew would not say how high the company was prepared to bid.  "It depends how they respond and what they want to do, but we'll work it through with them and make a decision," he said.

Marlborough Lines has 21,000 customers in the Marlborough region. Mr Dew said the offer was not"just a takeover", but an offer with both short and long-term benefits to the Eastern Bay of Plenty region.

But Mr Hennessy said community ownership was preferred when the issue was discussed last year, to protect against potential job losses in the region "if anything did happen."