Labor will seize on allegations that former Liberal MP John Alexander failed to declare rental income on his recently purchased luxury mansion in the New South Wales southern highlands, in the final bruising 48 hours of the Bennelong byelection.

Alexander purchased a $4.8m boutique horse stud at Moss Vale, Iona Park, in May this year, with the intent to use it for “short-term holiday rental and horse agistment purposes only”.

He declared the purchase of the property in parliament’s register of interests in June, then resigned from parliament in November after being caught up in the citizenship saga. He failed to declare any rental income on the property while he was still an MP.

Fairfax Media reported on Thursday evening that Alexander received rental income from the property while still an MP, but a spokesman would not say how much. “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]”, the spokesman said.

It is listed as available for rent on the Highland Holidays website for $1,440 a day.

Parliamentary rules state rental income must be declared on members’ register of interests, but there is variable compliance among federal parliamentarians.

Numerous Labor MPs, such as Victorian MP David Feeney, also own investment properties but do not declare rental income on their register of interest.

The candidates for Bennelong will move into overdrive over the two remaining days of the campaign for a seat which will determine whether the Turnbull government holds on to its majority in the House of Representatives.

On Thursday, Labor’s candidate Kristina Keneally, left open the option of taking the Senate spot vacated this week by Sam Dastyari in the event she loses Saturday’s byelection.

Pressed repeatedly on Sky News whether she would rule out taking the New South Wales Senate vacancy, Keneally said: “You can ask me any way you want. I’m not ruling anything in or out after the polls come in on Saturday night.”

Keneally is employed by Sky News as a broadcaster and she is currently on leave without pay to contest the seat. She said coming back to Sky “is definitely an option if I lose, but I’m here to win”.

“I’m fighting to win in this byelection,” she said. “I’m not even contemplating anything else.”

Both major parties are currently expecting Liberal John Alexander to prevail in Saturday’s byelection but activists on the ground also report substantial levels of voter frustration and disaffection. Two opinion polls published over the past week point to a line-ball contest on Saturday, or an Alexander win.

Given Bennelong has a substantial Chinese community, the contest has been complicated by the controversy surrounding Dastyari’s dealings with Chinese donors and the Turnbull government’s measures dealing with foreign interference – which have been met with a firm rebuke from Beijing.

In the event Labor loses on Saturday, insiders expect Keneally to take Dastyari’s Senate spot, although there are alternative candidates.

Malcolm Turnbull used a press conference on Thursday to make a direct pitch to Bennelong voters, warning the byelection was “a tight contest” and saying a vote for Keneally was a vote for putting Labor close to government.

The prime minister seized on favourable jobs figures released by the Australian Bureau of Statistics to argue his government was delivering for voters and he said voters in regional areas, where economic growth was less robust, “need to get the continued benefit of our leadership”.

“They need to get the continued benefit of the investment, the incentives that we provide,” Turnbull said Thursday. “John Alexander is part of a team that is delivering jobs and growth. His opponent, Kristina Keneally, puts all of that at risk.”

Keneally was also pressed on Thursday about her record in NSW politics, confirming she elevated Ian Macdonald, now in prison, to the ministry when she became premier.

“I did, and I say that quite upfront and I have always said that quite upfront,” she said. “I also sacked him and referred him for investigation”.

Keneally said her evidence before the Independent Commission Against Corruption “was a key part of finding convictions and the Icac has praised my evidence”.