The former TVNZ weatherman and surf lifesaver has dismissed claims he had anything to do with the missing money as "fantasy".
The New Zealand First MP has also brought in lawyers after an article was published revealing the allegations.
When asked if he considered stepping down from his party while the matter was being dealt with legally, Mr Horan replied "no, of course not".
"I can't comment anymore because it's in the hands of the lawyers now," Mr Horan said.
"I'm sure you can imagine what it's like."
Mr Horan said he did not know how long it would take before the matter was resolved.
"These things, I guess they take time."
Mr Horan's mother, Olwen Horan, 87, of Mount Maunganui, died in early August about three months after signing a codicil to her will, concerned that money was missing from her accounts.
Mr Horan's brother, Mana Ormsby, said the bank statements showed a large number of withdrawals from TABs and from ATM machines near Mr Horan's office in Mount Maunganui, Auckland and Hamilton. They also showed numerous payments at takeaways and video shops.
Mr Ormsby's initial investigation suggested about $85,000 was missing from his mother's accounts.
John Buckthought, the executor of the will, has declined to comment other than to confirm forensic accountants were examining Olwen Horan's estate and that she had won about $1 million in an Australian lottery some years ago.
Mr Horan said he wanted to thank the Tauranga community for their messages of support since the allegations surfaced.