Mr Speakman described Wagga Wagga-based upper house MP Wes Fang's position as untenable following a personal attack on the opposition leader in a now-deleted Facebook post.
"I have therefore terminated his appointments as shadow assistant minister for police and emergency services, shadow assistant minister for regional NSW, and shadow assistant minister for agriculture and natural resources, with immediate effect," he said in a statement.
"I have invited the leader of the nationals, Dugald Saunders MP, to nominate another Nationals MP to fill the vacated positions."
But the Nationals rejected the sacking, suggesting Mr Speakman didn't have the authority to remove him from shadow cabinet.
"As Wes is a member of the NSW Nationals, myself and my leadership team have formed the view that he is unable to be stood down in this way," Mr Saunders said.
"Therefore, Wes will continue to serve as shadow assistant minister for police and emergency services, shadow assistant minister for regional NSW, and shadow assistant minister for agriculture and natural resources.
"The NSW Nationals are committed to working with our coalition partners to ensure the best outcomes for NSW."
Mr Speakman issued another statement reiterating his stance and saying as opposition leader, he determined appointments to shadow ministry.
The internal stoush was sparked by a social media post from Mr Fang, accusing Mr Speakman of not notifying him about a visit to Wagga Wagga in which he met with local independent MP Joe McGirr.
"So ... the leader of the NSW Liberals Mark Speakman, slinks into #WaggaWagga, pretending like the Libs actually care about the Riverina," the post read.
"Did he ask the 'Coalition' member who lives in Wagga Wagga and is apparently part of his 'team' to have an in-depth discussion of Wagga Wagga issues?
"No. In fact, he didn't even have the courtesy to let me know he was coming."
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