Ms Wass said Cairns steered the conversation towards "spread betting".
When McCullum said he did not understand how this worked, Ms Wass said Cairns pulled out a piece of paper and explained the process.
Cairns is alleged to have said "everyone was doing it" and he didn't want McCullum to miss out, with payments of $70,000 to $180,000 per game.
Ms Wass also said Cairns explained how he was able to get money back to New Zealand without people asking questions, through an associate in Dubai.
McCullum found it difficult to decline immediately, said Ms Wass, as Cairns was his "idol" and he was "shocked" by the proposal.
In a later telephone call, McCullum allegedly told Cairns he did not want to be involved.
The pair met again later at a café in Worcester, UK, in June 2008, where Ms Wass said Cairns asked McCullum if he had changed his mind - he had not.
"Mr McCullum did not lodge an official report against Mr Cairns and his dishonest proposal at the time. He ought to have done so," said Ms Wass.
"He will tell you that he regrets that but explains it on the basis that Mr Cairns was a hero of his."
But McCullum was troubled enough to tell several people what happened, said Ms Wass.
They include his agent, Leanne McGoldrick, former Black Caps captain Daniel Vettori and former New Zealand teammates Shane Bond and Kyle Mills.
Ms Wass said former Australian captain Ricky Ponting remembers a conversation with McCullum, a teammate at Kolkata, which was interrupted by a phone call. McCullum said it was from Cairns about a business proposal.
Ms Wass said this was inconclusive evidence but dovetailed with McCullum's account.
It was not until February 2011 that McCullum formally reported the alleged approach, following an anti-corruption presentation by John Rhodes from the International Cricket Council's anti-corruption and security unit.
Ms Wass warned the jury that those witnesses would not provide independent evidence but what they would say was important because it shows McCullum mentioned the alleged approach "pretty near to the time of the approach itself".
"This is not something that Mr McCullum has made up recently out of spite. It was something that genuinely concerned him," said Ms Wass.
She told the jury that Cairns' legal team would say that McCullum is lying.
"You will consider that defence with care. You will ask yourself why a man like Brendon McCullum, who hero-worshipped Mr Cairns would invent such a lie?
"You will ask yourself why Mr McCullum would raise his head above the parapet to become involved in a case, where he knows he is likely to face challenge and to be discredited by Mr Cairns? The only possible explanation is he was telling the truth."
- Jared Savage in London
