Victorian contact tracers are in danger of being overwhelmed by a Covid-19 outbreak “running faster than we have ever recorded”, the acting Victorian premier James Merlino said as he announced a seven-day circuit-breaker lockdown.

Merlino said on Thursday contact tracers were struggling to keep up despite quickly identifying more than 10,000 primary and secondary contacts and 150 potential exposure sites, as the number of positive cases linked to the cluster reached 26.

“The usual transmission is about five to six days. In some of these cases, within a day it’s being transmitted,” Merlino said. “The number of cases has doubled in 24 hours. Unless something drastic happens, this will become increasingly uncontrollable.”

The B1617.1 variant, first identified in India, is responsible for Victoria’s cluster. While it is not the most infectious variant in India, it is nonetheless concerning, and at least as infectious as the B.1.1.7 variant prevalent in Britain.

“From first thing this morning, we have identified in excess of 10,000 primary and secondary contacts who will need to either quarantine, or test and isolate, and that number will continue to grow and change,” Merlino said.

“Our public health experts’ primary concern is how fast this variant is moving. We’ve seen overseas how difficult that movement can be to control. Here in Victoria, we’re seeing not only how quick it is, but how contagious it is as well.

“That’s the fastest our contact tracers have ever moved within a 24-hour period, identifying and locking down first, second and third rings of cases. But despite working as fast as this, and my thanks to all of our contact tracers, this variant is moving faster still.”

One elderly person with the virus is now in intensive care and on a ventilator, the acting premier said.

As of 11:59pm Thursday, the five reasons for Victorians to leave home include shopping for necessary goods and services; authorised work or permitted education; exercise; care giving; and medical reasons, including to get vaccinated. The lockdown will last for seven days.

Half of Victorians are now eligible to be vaccinated, as Merlino also announced those aged 40 and above can now book in for the Pfizer vaccine.

“The vaccine rollout has been slower than we have hoped,” Merlino said.

He added a lack of suitable quarantine facilities and a reluctance from some people to get vaccinated had been frustrating. Since the current outbreak in Victoria, however, record numbers of people have been vaccinated.

On Thursday morning, the vaccine booking website had crashed and the hotline was busy.

Asked if Victorians should feel angry at the federal government given the slow pace of the rollout and lack of suitable purpose-built quarantine, Merlino responded: “It is a fact that the commonwealth vaccine rollout has been delayed and there are global reasons for that delay.

“We have also seen a reluctance from some people to get vaccinated when they are eligible,” he said. “It is a fact that we have aged care facilities where not one person has been vaccinated. These are facts. We need to get this vaccination rolled out as quickly as possible, and that is why we have been doing a lot of work about how we can quickly expand what we offer at our state sites.”

The federal health minister Greg Hunt announced 130,000 vaccines would be released to Victoria to accelerate vaccinations in the state, including in the Whittlesea Local government area which has been particularly affected by the outbreak.

Victorian health authorities were also questioned about the state contact-tracing system and whether it had been too slow. The chief health officer, Professor Brett Sutton, said the suggestion was “absurd” and contact tracers had worked quickly.

“It is an absurd proposition that contact tracing has gone wrong,” he said. “We have talked about exactly what the contact-tracing team have done here, and it is extraordinary. Contact tracing is an integral part of how to get control. It does not do magic.”

Asked why the state was not locked down sooner, Sutton said: “We have to act on the information before us at any time.”

The chief medical officer Prof. Paul Kelly said it was clear there had been a super spreading event. In one workplace involved in the cluster, 10% of workers were now infected. “That points to high risk at this time in terms of transmission through the community,” Kelly said.

“My heart goes out to the people of Victoria, they have suffered through last winter, with a lockdown and so forth, I really hope this will be a short one and to get on top of this issue.”

Off the back of the outbreak, the Western Australia premier announced a hard border closure with Victoria as of 10am. “This means that travel from Victoria or travel to Western Australia from anyone who has been in Victoria from May 16 will no longer be permitted unless you are an exempt traveller,” he said. Tasmania declared the same, with the border closure taking effect from 2pm Thursday.

Queensland Premier Annastacia Palaszczuk said Victoria will be considered a hotspot from 1am Friday which means arrivals from the state will need to enter hotel quarantine. South Australia closed its border on Wednesday. The Northern Territory is requiring anyone who has been in Greater Melbourne and Bendigo to undergo mandatory quarantine.