Labor frontbencher Chris Bowen is refusing to say whether Bill Shorten has questions to answer over sweetheart deals with employers during his time as trade union leader.

Receipts and emails provided to the royal commission into union corruption show that in 2005, Mr Shorten’s Australian Workers Union Victorian branch invoiced a construction company for $38,228.68 to pay for 105 union memberships.

“The Bill Shorten I know … has devoted himself to improve people’s working positions,” Mr Bowen told ABC radio on Thursday.

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Prime Minister Tony Abbott described the allegations out of the royal commission as “pretty scandalous”, saying the opposition leader has questions to answer.

“Obviously these are matters that Bill Shorten is very, very familiar with,” he told Alan Jones on Radio 2GB on Thursday.

Mr Shorten has repeatedly said that neither he nor Labor will be providing a running commentary on evidence presented to the commission.

“In my time working as a union official, I have always put the interests of my members first every time,” Mr Shorten told reporters in Hobart on Wednesday.

“We don’t have any truck for corruption in the workplace, be it employer or union.”