From today, most employees will now be entitled to 10 days of sick leave per year.
The previous minimum allocation was five days if an employee had been at the job for six months.
Workplace Relations Minister Michael Wood said Covid showed how important it was for people to stay home if they were unwell.
"By giving people a minimum of 10 days sick leave, we’re helping them to do that and stop bugs from spreading," he said.
A person can also claim sick leave if their partner or a dependent child or someone who depends on them is sick.
PSA national secretary Kerry Davies said that 10 days of sick leave was "certainly not the end point, but it’s a great start".
"Anyone who has experienced an illness like cancer knows how fast sick leave and annual leave can dry up," she said.
"An ideal leave system would provide leave for workers as they need it."
National's Scott Simpson said that if "Covid-19 really is the reason Labour want the change to sick leave entitlements, then a better option would be to attach a sick leave response to the existing Covid relief package".
"The extra cost on business will inevitably end up being recouped by increasing the price of goods and services," he said.
Labour and the Green Party promised to double sick leave during the election campaign. Most employees were entitled to five days leave a year, which can be carried over to a maximum of 20 days.
Simpson said last November when the changes were introduced to Parliament that an economic crisis was "not the time to be loading more costs onto businesses in the form of extra sick leave".
"Doubling sick leave just piles more costs onto business at a time when they can least afford it, coming on top of minimum wage increases and the proposal for an extra public holiday," Simpson said.
During the election campaign, then Labour workplace relations spokesperson Andrew Little said 35 per cent of Kiwis turn up to work every year sick and "managing Covid-19 has shown, more than ever, how important it is for workers to be able to stay home if they are sick".
Extensions to sick leave could help ensure employees can "fully recover" from their illness before returning back to work, Little said.
Earlier this year, the Government signalled new changes were on the way to leave entitlements, including expanding the criteria of bereavement leave to include more family members and giving employees a sick day from the first day of employment.