Victoria's fifth lockdown will be extended after the state's outbreak grew by another 13 community cases today.

"We will not be ready to remove the lockdown at midnight tomorrow night," Victorian Premier Daniel Andrews announced this afternoon. "It would be perhaps a few days of sunshine, and then we'd be a very high chance to be back in lockdown again, that's what I'm trying to avoid."

Andrews did not specify how long the lockdown extension would be, saying "as soon as [health officials] can confirm for Victorians how much longer [the lockdown will last] and in what circumstances, I will do that. And I hope that will be around this time tomorrow."

There would be an ongoing challenge in defending the border against the outbreak in NSW, Andrews said, and the hard border between the two states could remain in place for some time.

Victorian health chief health officer Brett Sutton said while the signs of controlling the outbreak were good, there was still a huge risk if the Delta variant stays in the community.

"It is not a question of locking down for a short period of time versus life as normal. Life as normal puts at risk hundreds of cases becoming thousands of cases, becoming tens of thousands of cases," he said.

"These are awful and difficult decisions, but we cannot pretend that it is simply a choice of locking down or not having to worry about it. This will take off if we allow it to. We have to extinguish each and every chain of transmission."

Andrews said the snap lockdown was the right decision, claiming "we know that if we had been open, we'd be just like Sydney with hundreds and hundreds of cases ... lifting restrictions now would not be the right thing to do."

Melbourne's growing outbreak

The expanding list of exposure sites has now topped 250 venues, including several bars and restaurants in Melbourne's inner north, South Melbourne Market stores and a CBD McDonalds.

Several train services were also added, along with a number of venues in Mildura in the state's northwest after a positive case was detected there yesterday.

Andrews said contact tracers had so far been able to link all new cases to existing clusters, meaning the outbreak was "unfolding as hoped".

"The fact we're finding people and getting to people as quickly as we are is clear evidence of how hard our contact tracing team is working … they're doing a great job for all of us," he said.

However, the expanding list of exposure sites meant the government was not in a position to say when lockdown restrictions would lift - even in regional areas.

"I know it's frustrating, but this virus can reach you as we can see in this Mildura case," he said. "I will not release any area of the state [from lockdown] unless I have advice to do so. I do not have that advice.

Melbourne is now in its fifth lockdown. Photo / Getty Images

Public health expert Professor Bill Bowtell said the trajectory of Victoria's outbreak showed that the state's snap lockdown was more effective than NSW's method.

"The [cases] are all linked, so that is a tremendous achievement," he said. "Of course, they went into the lockdown to bring about this type of outcome. Short and sharp. And let's hope it can end very soon.

"I am sure they don't want what we are going through in Sydney now, which is this long, drawn out process to come to the inevitable conclusion that we should have moved quicker and faster and harder sooner."

Quarantine-free travel from Victoria to New Zealand was paused at 1.59am (NZT) on Friday.

Covid-19 Response Minister Chris Hipkins said that decision followed updated public health advice from New Zealand officials.

The pause would run for at least four days from 1.59am (NZT) July 16 and be subject to further review today.