BHP Billiton’s earnings could decline by as much as 14 per cent in 2014-15 and a long-expected buyback could be delayed as the deteriorating iron ore price hits the resources group's earnings.

BHP will release its fourth-quarter production results on Wednesday.

Expectations of a buyback or some other capital management exercise were building at the start of 2014. Then came the dramatic fall in the iron ore price of almost 25 per cent to about $98 a tonne

Credit Suisse analyst Paul McTaggart says the prospect of sharply lower earnings in fiscal 2015 have all but ruled out a buyback or special dividend.

"We think capital management initiatives [other than a lift in the progressive dividend policy] will be deferred until August 2015," he said.

“If there is a buyback, it will likely be small [less than $3 billion] and not material in terms of BHP’s scale."

UBS analyst Glyn Lawcock believes BHP could still announce an on-market buyback when it unveils its full-year results in August as it stays on track to pare net debt to about $25.3 billion.

"We expect any announcement to be an on-market buyback with any off-market buyback delayed until there is an adequate buffer between the net debt level and BHP's targeted net debt level," Mr Lawcock wrote in a note.

Analysts say it is not the quantum of a buyback that is key but the buyback mechanism BHP will employ.

Deutsche Bank analyst Paul Young says the market "is too hung up" on the dollar figure of a buyback, and says BHP could opt for an on-market rolling buyback mechanism.

"If it’s a $3 billion buyback or a $5 billion buyback that really isn’t going to move the needle for a $200 billion company," he said. "The market is too hung up on it.

"It’s the mechanism that they choose that is important, much more than the amount."

BHP is expected to unveil a rise in June-quarter iron ore production when it releases its figures midweek as its production nears 220 million tonnes a year in WA.

Mr McTaggart is tipping fourth-quarter iron ore shipments for BHP of about 62 million tonnes, and says full-year shipments will be 223 million tonnes, well ahead of the guidance figure of 217 million tonnes.

But he expects a lift of about 5 per cent in BHP iron ore volumes as fiscal 2015 begins, and says further cost reductions amounting to about $4 per tonne will not be enough to offset the "massive" revenue hit likely from the lower iron ore price.

Mr McTaggart tips record iron ore earnings before interest and tax of $11.8 billion for the year just ended, and a dramatic fall to $7 billion in fiscal 2015.

He said a forecast average price of $US91 a tonne in fiscal 2015, from $US123 a tonne the previous year, would push group earnings for BHP 14 per cent lower in fiscal 2015 and "bring added scrutiny regarding its premium rating".

Credit Suisse estimates the falling iron ore price will reduce BHP's margins of $59 a tonne for the financial year just ended to $33 a tonne in fiscal 2015, before they bottom out at $30 a tonne in 2016, when iron ore prices are expected to bottom.

BHP's overall underlying EBIT would fall to $20.3 billion in the period, from $23.6 billion, before gradually again rising back to about $23 billion by 2018, Mr Mctaggart said. The prediction was well below consensus EBIT of about $22.4 billion. He said the consensus was "hard to reconcile".

"Into financial 2015 we expect that volume growth will moderate, commodity prices will decline more sharply and the rate of pace of unit cost reductions will slow. The net impact should be sharply lower after-tax profit for BHP’s mining operations."

This sits in contrast to Credit Suisse’s forecasts for Rio. Its profit is tipped to grow from a forecast $8.9 billion in calendar 2014 to $9.6 billion the following year.