01/11/2016 11:20 AM AEDT | Updated 01/11/2016 12:41 PM AEDT

'I Am Sorry It Has Ended This Way'

Family First's Bob Day has formally resigned from the Senate, effective immediately.

Alex Ellinghausen, Fairfax
Family First Senator Bob Day says it was an

CANBERRA – Less than a week after he insisted there were too many important pieces of legislation for him to be able to leave the Senate early, Family First's Bob Day has formally resigned from the Senate.

The crossbench Senator, whose national building empire has collapsed with hundreds of homes unfinished, has sent a letter of resignation to Senate President Stephen Parry, asking that the move be regarded as "effective immediately".

In a statement, Day cited the withdrawal of a major investor for his decision.

"While a number of offers for various parts of the Home Australia business have been received, the major investor who has been examining the Group's portfolio of assets over the past fortnight, has decided not to proceed," Day wrote.

"Accordingly, I have today tendered my resignation to the President of the Senate effective immediately."

The construction union, CFMEU, which was preparing to challenge the legality of any vote Bob Day would cast over the Government's ABCC legislation, is celebrating.

The South Australian said it had been an "honour and a privilege" to serve in the Australian Senate, but added his political demise had left a sour note.

"I am sorry it has ended this way," Day declared.

Day had already flagged he was going to resign but six days ago, he tweeted that the "marriage plebiscite, ABCC and our other work" was "too important" to allow Family First to have a vacant seat "even 1 day in November."

Attention now goes to who will replace Day in the Senate but the timing, earlier indicated by Day, is likely leave the Senate with a number short.

Day's now former Chief of Staff, Rikki Lambert, has confirmed he is in the running to replace his former boss in the Senate.

Lambert has been unavailable for comment.

As for Day, he's now promised to devote his time and energy to assisting home owners affected by his company's closure.