"When I hung up I literally did not see that that was anything other than what a member of parliament would normally do on behalf of somebody who had asked.
"In 26 years as an MP when I have hung up the phone from a call to ACC or the police or the health board advocating on behalf of somebody I've always thought that was my job and I wasn't crossing a line.
"However it has become clear that the police believe that it does cross a line, the Prime Minister thinks that it was inappropriate for me to have made the call."
He said he was told of the December incident by a friend of Liu and was "shocked" due to Liu's clean record required for citizenship here.
He said he was told by Mr Liu's interpreter there was confusion over the incident.
"I said I would find out from the police what the status of all this is and has it come to an end."
Williamson said he was not "friends" with Liu.
"It is pretty hard to have a friend that you cannot speak a word of their language and they of yours.
"I don't socialise with him."
Mr Williamson said he was feeling "pretty much shattered and pretty gutted".
"My family's in a bit of tatters".
"There is clearly a perception that a member of parliament should not call the police at all about a case and I will make sure I will never do that again.
"We are here because of me having resigned, I have admitted I made an error of judgement."
It was normal conduct for him to advocate for people who approached him, he said.
"Over the course of 27 years, I would have called the police four or five times a year."
He brought up the financial holdings of Mr Liu as "background" for the police, he said.
"I said he was a very large investor in the building and construction sector...as a bit of background to who he was.
"I have a perception that we economically need a huge level of investment above what we have now."
He also said he took domestic violence seriously.
"I know about domestic violence, I am a big supporter of Women's Refuge here and I have raised money for them here."
About 20 members of the media greeted Williamson at his Pakuranga Rd office.
Shortly before the conference began, Jamie Lee Ross arrived and told reporters, "No comment".
"I'm just here to support Maurice - he's a mate and a colleague."
He then went inside the office.
Read more of the Herald's coverage of this story today:
• Emails between top cops revealed
• Maurice Williamson resigns as a minister
• Audrey Young: Williamson's 'significant error of judgment'
• Profile: Who is Maurice Williamson?
Read a full statement from Mr Williamson below:
In late January I was made aware that a Chinese businessman who had been granted residence by Labour and who I had helped get citizenship, had been involved in a domestic dispute in early December.
When I made enquiries with his associates it became clear that there was confusion about whether a prosecution would proceed.
I offered to call the police and clarify the status of the matter.
When I spoke to police in January I used phrases such as "Can I just start by making it clear, because I do want to make it really clear - In no way am I trying to force a decision of any sort."
I thought that I had made it crystal clear I was simply enquiring as to the status of the matter. The police seemed fine with the conversation and even invited me to give them another call if there was any further business.
It is clear to me now that that was an error of judgement and regardless of what I actually said - it could be interpreted I was trying to influence the issue.
I have given the Prime Minister an absolute assurance I was not trying to influence police processes and I was only doing the normal job of an MP. However, it is clear that a perception of my trying to influence the outcome has been created and for this reason I have tendered my resignation as a minister today.
