Del Mastro has pleaded not guilty to charges of overspending during the 2008 campaign, failing to report a personal contribution of $21,000 to his own campaign and knowingly submitting a falsified document.
He is expected to take the stand later today at his trial in Peterborough, Ont.
Earlier, defence lawyer Jeffrey Ayotte asked his client's former adviser, Alan Wilson, about the Peterborough Conservative Electoral District Association's dealings with Holinshed Research — the firm whose services lie at the heart of the case.
The Crown alleges Del Mastro paid the firm for its voter identification and get-out-the-vote calling services from a personal account, thereby exceeding a limit on personal contributions for the election campaign.
The Crown has also alleged Del Mastro tried to cover up the overspending by using backdated invoices to make it appear Holinshed had only charged a fraction of the overall cost during the election period.
But under questioning from Ayotte, Wilson said he thought Del Mastro had only paid Holinshed for its new software, called Geovote, designed to make it easier to identify the political leanings of those living in a riding.
Wilson says the Peterborough EDA, of which he is now the president, had "at one point in time" paid for a portion of Geovote.
He also says Del Mastro's constituency office, where he used to work, received a sample of Geovote but was unhappy with the way it performed and never sought the full version of the software.
"What is your understanding that Holinshed had been paid for?"Ayotte asked.
"For Geovote," Wilson answered.
Del Mastro left the Conservative caucus in September 2013 and has maintained his innocence throughout the trial, which is now in its third week.