A notorious Perth bikie has changed his plea to guilty midway through his trial on charges of repeatedly assaulting his former partner.

Finks bikie Troy Mercanti had originally pleaded not guilty to five charges of bashing Tammy Kingdon during their 15 year relationship.

However, today at the start of proceedings and in front of a jury, Mercanti admitted four of the charges against him.

The fifth charge was dropped by prosecutors.

The plea change came after evidence from Mercanti's mother who testified she saw her son strike Ms Kingdon at a family gathering on Christmas Day in 2006.

Ms Kingdon had already told the court she was repeatedly punched and kicked by Mercanti that day after she complained he had not been home for three days.

His lawyer suggested she was lying.

Mercanti's lawyer Shane Brennan says Mrs Mercanti's evidence led to the change of plea.

"His mother gave evidence which was damning and our client felt compelled to review his position," he said.

"He pleaded guilty on the basis that the state drop one of the charges and he pleaded guilty to the rest."

His lawyers told the District Court the evidence left Mercanti shaken.

Mr Brennan says his client expects to receive a substantial jail term after changing his plea to guilty.

Mercanti has been remanded in custody until a sentencing hearing on Wednesday.