Greens leader Richard Di Natale failed to declare his family farm in Victoria's Otway Ranges for 15 months, breaching parliamentary rules and potentially placing him in "serious contempt" of the Senate.

And Senator Di Natale has paid three au pairs to help with his family as little as $150 a week after tax, or $3.75 an hour - based on a standard 40-hour week - as well as room and board worth $300 a week.

He says he made up the difference and paid above minimum wage requirements [based on advice from a payroll services company] and by requiring only 25 hours of work a week.

The Greens leader has made workers' pay and conditions, and a promise to protect penalty rates, a central feature of the 2016 election campaign.

He has attacked Bill Shorten over the penalty rates issue too after the Labor leader vowed to respect the rulings of the independent Fair Work Commission and said he would not - unlike the Greens - legislate to protect penalty rates.

Senator Di Natale and his wife, Lucy Quarterman, jointly own a 20-hectare farm "Twin Gums" and a second investment property in North Melbourne, which is not negatively geared, when he entered the Senate on July 1, 2011.

He declared ownership of the North Melbourne property on his Register of Senators' Interests on July 28, 2011 but omitted ownership of the family farm, which was his primary residence - breaching Senate rules about the disclosure of property owned solely or jointly by a senator.

In October 2012, the GP-turned-MP transferred the jointly owned farm to Ms Quarterman's name, removing the requirement for it to be declared publicly on "Form A" of the Register.

But between July 1, 2011 and October 2012, he was joint owner of Twin Gums, which means that like Labor MP David Feeney, who Fairfax Media revealed on Tuesday failed to disclose a $2.3 million property on the Register, Senator Di Natale could be found to be in "serious contempt" of the Senate under disclosure rules, and censured.

Senator Di Natale's spokesman said "the farm was listed as a business interest from the time Richard was elected" and claimed that by declaring income from the farm, Mr Di Natale had met requirements. However, the senator has clearly breached the rules as stated by the Senate, which require all property solely or jointly owned by a senator to be declared.

The 2012 post on backpackerjobboard.com.au by Ms Quarterman, advertising an au pair job paying $150 a week, plus food and board, is potentially more embarrassing.

The ad stated the family of four is "looking for an extra pair of hands around the place to entertain the lads [the couple have two boys] and help with cooking and general domestic duties".

It adds: "Will take couples but weekly wage remains the same."

The $150 weekly wage was a quarter of the national minimum wage in 2012 of $606.40 per week, or $15.96 per hour.

A couple working for $150 a week would be earning just $1.88 per hour.

A spokesman for Senator Di Natale said the employment conditions of the au pair hired in 2012 were "based on advice received" that a deduction of $300 for food and board was appropriate and that the au pairs worked approximately 25 hours each week

"PAYG tax was also paid, at $37 per week, which brought the gross figure to $187, which was above minimum wage," the spokesman said.

"The au pairs worked during business hours (while Lucy worked) so penalty rates did not apply. Advice was that Superannuation was not payable."

The Fair Work Ombudsman states au pairs are entitled to the national minimum wage but can trade away food and lodging if both parties agree in writing.

The tax office states an au pair employed for more than 30 hours a week must be paid super.

Subsequent au pairs, including "Ben" - featured in a 2015 Good Weekend profile - were "paid above minimum wage based on the same conditions and a bonus was also given at the conclusion of his term. He remains a close friend of the family", the spokesman said.

Senator Di Natale has challenged Labor: "If you are so committed to penalty rates, protect them in law...if you care about penalty rates as the Greens do, then join us and let's protect them in law."

He also attacked Mr Feeney for blaming the "maelstrom of events" for his failure to declare his $2.3 million Northcote property.

"I think he's got some serious answers to give and so far the explanation he's given falls well short . . . I can definitely say that I haven't purchased a $2.3 million property - I reckon I would have remembered that," he said.

Follow James Massola on Facebook.

Follow us on Twitter