Mr O'Connor said he did not regret letting Liu into the country.
"In all those cases you make your decision based on the information in front of you and move on. Because there are so many of them, it's only the exceptional ones you recall and quite frankly this wasn't one of them."
Mr O'Connor said Mr Williamson crossed the line when he rang police about the Liu case.
"It's not protocol, it's not proper. I think every politician is aware of the sensitivity around it and respects that The Prime Minister has made the judgment on this one and I don't think he's wrong."
Mr O'Connor said he could not recall ever contacting police himself about individuals facing prosecution.
"But I've rung police for general policy decisions. I've discussed with police how they make general calls on who to prosecute and who not to, in reference to - I think it was a tourist driving incident.
"But I certainly haven't done it for anyone I know or had the details of the case."
He said it was Mr Williamson's call whether he resigned as an MP.
Mr Williamson also lobbied for Liu when Liu sought to become a New Zealand citizen. Then-immigration minister Nathan Guy subsequently approved Liu's citizenship, against official advice.
Liu has since reportedly donated $22,000 to the National Party.
- Westport News