The ACCC reported in August that nothing dodgy was going on with fertiliser prices even though they have near trebled in some cases in recent years.

But the Senate Select committee which has been investigating the matter since the start of the year isn't convinced.

Chairman Bill Heffernan says he wants the ACCC to explain how it can say there is no collusion or cartel behaviour taking place in the fertiliser industry when the committee has received evidence to the contrary.

Senator Heffernan said his committee has received in camera evidence from people who've felt too intimidated by the major fertiliser companies to appear publicly, but have said privately they were appalled at the events of the past 12 months, especially in relation to "hoarding and gauging" he said.

"I'll be interested to hear what the ACCC has to say because we have got the proof to show us there's a global monopoly on rock phosphate," Senator Heffernan said.

"Any company that has 70pc of fertiliser sales and 100pc of manufacturing would appear to have a monopoly power," Senator Heffernan said.

At a conference in Sydney last week, Senator Heffernan said suppliers have expressed surprise at the unjustified fortnightly increases in prices, while there is now evidence of increasing supplies up to 2011 and a sharp drop in prices over the next year.

In its report to Government in August the ACCC concluded international supply and demand factors only were to blame for fertiliser costing so much.

The findings were made during a broader investigation into the rising cost of groceries, but are at odds with evidence being gathered by this Senate committee.

Already the inquiry has exposed specific cases of market manipulation, market abuses because of too few players in the market and the export of Australian fertiliser to countries such as Brazil which has exacerbated the domestic supply shortage.