Mr Horan told Radio New Zealand today that many of the calls to the TAB were made while he was with his mother.
"Many of the phone calls were during December and January, when most of the time I was with my mother and we were sitting down and watching the television, and the horses go by and she'd say, 'Gosh I like the look of the that one'."
Asked whether he used his mother's account without permission, he said: "Don't be ridiculous."
Mr Horan said he would stand in the next election but it remained to be seen if he would run as an independent.
"It's quite simple really, I'm now an independent MP and I've done nothing wrong, so why should I leave?"
NZ First leader Winston Peters has said Mr Horan's use of his parliamentary cellphone to place bets was one of the reasons he sacked the MP.
Before the NZ First board announced its decision, Mr Horan said he still considered himself a member and would be disappointed if he was expelled.
"I would expect that they would call me for my version of events and I haven't been called so I'm not going to pre-empt their decision that may or may not be made."
However, he would not challenge any decision to expel him.
Mr Peters last night said Mr Horan ceased being a member of the party when he approached the Speaker, Lockwood Smith, last Wednesday about staying on as an independent MP. "Under our constitution that's the end of it. It's a fait accompli achieved by his own request."
Mr Key said yesterday that an MP's personal use of a parliamentary mobile phone was not a reason to sack them.
"I use my mobile phone to ring my wife, I use my mobile phone to make restaurant bookings."
Mr Key said he understood that was permitted as a fringe benefit.
He also questioned whether the frequency of Mr Horan's calls to the TAB indicated he had a gambling problem.
"Ringing 14 times a month - is that an addiction? I reckon that would be a stretch."
However, Mr Horan is under fresh pressure in the form of media reports that forensic accountants examining his late mother's affairs have raised issues about a number of cheques.
He responded yesterday by swearing on a Bible provided by a reporter that he did not have a gambling problem and had never stolen money from his mother.
However, he did not deny receiving money from her.
"My relationship with my mother was such that if I ever required anything all I needed to do was ask", he told RadioLive's Duncan Garner.