The High Court in Auckland ruled against a request from rival bidder Peak Petroleum to delay a March 6 vote on Shell's offer, but Peak a consortium of investors and smaller oil companies, responded by upping its offer to 15 US cents per share - valuing FCL Energy at around NZ $5.1 billion ($2.2 billion).

Peak, which also lost a plea for court-sanctioned access to inspect FCL Energy's accounts, is now offering US $3.85 per share in cash for FCL Energy, along with the issue of shares in related companies Capstone and Rubicon.

Shell, in conjunction with Houston oil company Apache Corp on Thursday, upped its cash offer to $3.55 per share - or around NZ $4.8 billion - from a previous $3.34 per share. It too is offering to issue Capstone and Rubicon shares to FCL holders.

The Peak consortium includes small unlisted NZ oil firm Greymouth Petroleum, investment group Guinness Peat Group Plc and Canadian oil company Penn West Petroleum.

The NZ Director of Guinness Peat Tony Gibbs, said the next move could be a court appeal, but in the meantime the race was on to tie-up proxy votes ahead of Tuesday's shareholder vote.

"Let's see where we go from here - we've upped the price on Shell, what are Shell going to do," said Mr Gibbs.

But Fletcher Challenge said it has yet to see a formal offer document from Peak, and it called the group's bid "illusory".

"The mentioning of enticing-sounding figures is only likely to mislead some shareholders into believing that an offer has been made when, in fact, none has," FCL Chief Executive Michael Andrews said in a statement.

"If there was any possibility that (Peak) did have the capacity to deliver a credible offer with certainty for shareholders, and without risking losing the Shell and Apache offer, we would be actively courting them."

The court backed the view of Fletcher Challenge Ltd - FCL Energy's parent - that the matter was for shareholders to decide at Tuesday's scheduled meeting.

Judge Anderson said it appeared Greymouth, which appeared on behalf of Peak, was attempting to use the court, to obtain a commercial result in the expectation, that delays in the scheme of arrangement would squeeze out Shell and Apache.

Shell and Apache made it plain, that they would walk away from the deal if their long standing deadline of March 23 was missed.

Earlier, shares in FCL Energy closed up 10 cents at NZ $9.13 on heavy turnover of 9.9 million shares worth NZ $91 million.

In the past three days - since Peak entered the race -around 10 per cent of the stock in FCL Energy, has changed hands with brokers saying some investors were keen to benefit from a bidding war, while others were cashing up earlier profits.

The FCL Energy sale is the largest, remaining component of the break-up of the group - with shareholders also due to vote on Tuesday on the separation of its other divisions, FCL Building and FCL Forests .