Accused tax fraud conspirators Peter Bartlett and Ron Sayers were more interested in blasting and drilling than filling out and filing tax returns, a Supreme Court jury has been told.

Defence lawyer David Staehli said the wealthy mining contractors and lifelong friends entrusted complex legal and financial matters to advisers, including Barminco director turned rollover witness Trevor Thomson.

Mr Staehli said Mr Thomson had failed Mr Sayers and Mr Bartlett in his role as their accountant and had betrayed their trust. "He was unreliable, incompetent and lazy," the barrister said.

Mr Thomson has been subject to scathing criticism over the past three days as defence lawyers began summing up after an eight-week trial over an alleged conspiracy to cause a loss to the Commonwealth.

Mr Bartlett, Mr Sayers, tax scheme promoter Greg Dunn and Mr Thomson's former protege Deborah Grace have pleaded not guilty to conspiring with Thomson over a $7 million false interest bill created for the Bartlett and Sayers family trusts in 2002.

In 2010, Mr Thomson pleaded guilty to conspiring to cause a loss to the Commonwealth.

He received a 39-month sentence with a 13-month minimum. His term was cut from a head sentence of eight years for his co-operation with Australian Crime Commission investigators, his guilty plea and agreeing to be a Crown witness, the jury was told.

Mr Staehli, who is leading Mr Sayers' defence, said his client signed documents presented by advisers, relying on them to give him a broad understanding.

Prosecutors claim the false interest rort was explained to Mr Sayers and Mr Bartlett in 2002, but the contractors cannot recall an alleged briefing and say they would have objected to anything involving wrongdoing.

The false interest transactions were allegedly contrived to protect a $50 million tax avoidance scheme through which Mr Sayers and Mr Bartlett slashed the tax on their profits from Barminco, an underground mining contractor then run by Mr Bartlett.

Mr Sayers is the longstanding managing director of listed surface contractor Ausdrill, which had looked at merging with Barminco last decade.

The jury has heard that some of Mr Sayers' closest associates, including Ausdrill chairman Terry O'Connor and lawyer Ian Cochrane, had been unaware of his silent half share of Barminco.

Mr Staehli said yesterday prosecutors had tried to cloud evidence of Mr Sayers' good character by pointing to the secrecy surrounding his Barminco stake.

He said it had been fully disclosed to the Australian Taxation Office in tax returns showing Barminco profits flowing to Sayers family trusts.

Mr Staehli said Barminco auditors had not raised concerns when handed a document in which a mention of Mr Sayers' stake was blanked out.

"It was private because it was Mr Sayers's private business," he said.

Mr Bartlett's legal team and Mr Dunn, who is representing himself, are due to sum up next week ahead of Justice Eric Heenan giving final directions to the jury.

The West Australian

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