It has been almost 11 weeks since a sexual complaint was laid against Darren Hughes, a Labour MP at the time, but there's no sign of when the police will wrap up their investigation.

The length of the inquiry has led some to question why it has taken so long, but police insist they are simply conducting a thorough inquiry.

The complaint relates to an alleged incident in Wellington in the early hours of March 2 involving Mr Hughes and an 18-year-old student after an evening of socialising following a debate at Victoria University.

Mr Hughes and the complainant ended up at the home of Labour deputy leader Annette King, where Mr Hughes boarded when in Wellington, and some hours later, witnesses reported seeing the complainant naked in the street.

One Labour MP told the Herald there was some frustration at the length of time the investigation was taking.

"It's unfair on them both, but particularly on Darren, because his life's on hold."

Criminal lawyer Greg King said a sexual complaint was usually "dealt with very quickly, especially a contemporary one rather than a historical one going back several decades".

"Whether there are some complicating factors that we're not aware of, I don't know, but it does seem to be dragging on, and it's putting a lot of lives on hold in the interim."

Mr King rubbished the suggestion that political sensitivity might prolong an investigation.

"A core principle of the system is what we call blind justice - that it applies to everyone equally regardless of social status or occupation or anything else."

He said the fact that the complaint involved two males should not present any "logistical or pragmatic difficulty or reason for a more prolonged investigation".

Bill Hodge, an associate law professor at Auckland University, said it was a concern that an investigation could drag on for months, but "in my experience these things always take longer than I think they should".

Gathering and analysing forensic evidence in particular could take some time.

"Police are simply taking all due care," Professor Hodge said.