"Because we still have a lot of people who aren't yet fully vaccinated, that level of cases would mean a large number of people needing hospital treatment."
Of the 158 people in hospital since the start of the Delta outbreak, only three people were fully vaccinated.
Plank said it is "very clear" the current Covid-19 outbreak is growing, with all indicators trending upwards.
Where to get a vaccination in Auckland - without a booking
"All the key indicators have been trending upwards in the last two weeks, including total cases, unlinked cases, cases infectious in the community and the number of contacts."
While vaccination rates will help slow transmissions down, Plank believes there is a danger too many cases will lead to the contact tracing system struggling.
In Auckland, 87 per cent have had their first dose and 63 per cent their second dose.
There were 69,118 vaccines administered yesterday, 15,107 first doses and 54,011 second doses.
"This means we need to do everything possible to minimise community transmission, particular in the next few crucial weeks as we get more people double-dosed."
When asked if there has been an impact of Step 1 in Auckland's road map, Plank said it is too early to tell but believes a small increase could "spell a big trouble".
On Tuesday, 43 new community cases were announced. Director general of health Dr Ashley Bloomfield said 40 of today's cases are in Auckland and three are in Waikato.
Prime Minister Jacinda Ardern has announced a televised' "vaxathon" this Saturday, which she hopes will break daily records for Kiwis getting the jab.
It will include real-time data on how regions are going, prizes, information from experts and competition between towns and regions.
This Saturday multiple providers open for longer hours to enable more jabs to be administered.