Mr Alexander, the Turnbull government's candidate in the crucial Bennelong byelection this Saturday, bought the sprawling 100 acre property in May after inspecting it with his soon to be neighbour, radio broadcaster Alan Jones.

In keeping with Parliament's rules, he declared the purchase on June 20, 2017 and at the time, told Domain he would use the Moss Vale property for "short-term holiday rental and horse agistment purposes only".

However, the former tennis professional and media commentator has never declared a dollar of income from renting out the home, despite Parliament's register of member's interests specifically requiring members of parliament to declare "any other substantial sources of income".

The estate is available for rent on websites including Highlands Holidays and Stayz for $1440 per day and it is nearly fully booked for December and January; for the price, guests have access to a swimming pool, five bathrooms, a billiards room, media room, formal library and outdoor entertaining areas.

Fairfax Media has confirmed he received rental income from the property but Mr Alexander would not disclose how much.

In a boost for Mr Alexander, a Fairfax Media-ReachTEL published on Thursday revealed he led Labor's Kristina Keneally in the all-important two-party preferred vote 53 per cent to 47 per cent.

But the revelation he breached the rules while in Parliament – just two days before the people of Bennelong vote on Saturday – will be deeply embarrassing for the Liberal candidate.

In a statement, a spokesman for Mr Alexander said: "John Alexander declared the purchase of a property, to be operated as a business, in a timely manner and in line with the advice from the Clerk [of the House]".

Numerous other government MPs – including Prime Minister Malcolm Turnbull, David Gillespie, Warren Entsch, Ian Goodenough and Barry O'Sullivan – own investment properties and do separately also declare rental income from their real estate investments.

However, at least three other MPs – Labor's David Feeney, Brendan O'Connor and Kate Ellis – also own investment properties but do not separately declare rental income, in what is also an apparent breach of the rules.

MPs have 28 days to update their register when their circumstances change; Mr Alexander had six months to update his register before he resigned from Parliament on November 11 because of another paperwork failure, on that occasion the fact that he was a dual British-Australian citizen.

Any member who fails to keep their register up to date can be found to guilty of serious contempt of the House of Representatives. In practice, that means Mr Alexander could have been referred to the House Privileges committee and censured while he was in Parliament.

If Ms Keneally were to defy expectations and win Bennelong for Labor, Mr Turnbull would lose his majority in the Parliament, placing his government in a more precarious position.

In addition, Labor and the crossbench would have the numbers to team up and refer government MPs to the High Court, as they attempted to do in the last sitting week of the year.

The battle for Bennelong has become a bare-knuckle fight, with the major parties trading blows and making claims and counter-claims about health care funding cuts, damage being done to the China-Australia relationship by proposed foreign interference laws and Ms Keneally's links to disgraced Labor figures such as Eddie Obeid.

Mr Turnbull slammed suggestions from Labor that he was "anti-Chinese", declaring there were "one million Australians of Chinese ancestry, among whom is my own granddaughter".

"I think it is desperate, demeaning, and it is disappointing. The people of Bennelong can see through all of that. And they have a very clear choice, they have a man, John Alexander, who has been their champion."

Ms Keneally, for her part, three times failed to rule out taking the NSW senate seat that will soon be vacated by Sam Dastyari, who resigned earlier this week.

"And as I've said before, I've been offered Senate seats before and turned them down. This is all I'm focused on, Bennelong, no one has offered me the job and I haven't had any conversations about it," she said.

Labor insiders in NSW believe Ms Keneally is the front-runner to take the senate seat if she does not win Bennelong, though candidates such as United Voice union leader Tara Moriarty and the Lowy Institute's Michael Fullilove have been discussed.

Mr Alexander is not the first MP to be caught failing to declare property on Parliament's register of members interest; last year, Fairfax Media revealed Mr Feeney forgot to declare ownership of a multimillion-dollar home in Melbourne.

A senior Labor source said the ALP would consider referring Mr Alexander to the privileges committee if he wins the election on Saturday.