Wellington's Mt Victoria Tunnel has had its opening delayed by a poo on the road.

The New Zealand Transport Agency will review its response times after taking 45 minutes to reopen Wellington's Mt Victoria tunnel when a drunk man defecated on the road.

The busy commuter tunnel, which forms part of State Highway 1 through the city, was scheduled to reopen at 6am on Thursday after being closed for routine maintenance overnight.

But just as it was about to reopen, a man jumped down from the walkway and answered the call of nature.

A cleanup crew had to be called in, delaying the reopening until 6.45am.

NZTA regional performance manager Mark Owen said the incident happened after the overnight tunnel crew had packed up, and another crew had to be called in to deal with what he called a hygiene and road safety issue.

"At the end of the day, this is an unusual and isolated case in which an irresponsible act resulted in a slight delay to a scheduled tunnel reopening," Owen said.

"We'll be looking into whether response times could be improved, although we're hoping there isn't a next time."

"Effluent of any kind can be dangerous to road users like motorcyclists, as it makes the road surface slippery. This can be especially hazardous inside a tunnel, and we had a responsibility to ensure that the road surface was safe before opening the tunnel to motorists," Owen said.

Owen said the tunnel was not closed because of the incident - rather its opening was delayed and it was open well before 7am.