Arguments for and against the Employment Relations and Holiday Amendment Bill have been heard at a select committee meeting in Auckland.
The bill proposes an extension of the 90-day trial to all businesses and would restrict union access to workplaces.
Robert Reid from the National Distribution Union was the first to make a submission at the hearing on Wednesday.
Mr Reid argued the current laws work well and the 90-day rule will stop people trying to better themselves.
"Instead of making the labour market more flexible, which is what the Government and employers say they want, it'll actually make it more rigid.
"What workers are telling us is that they are less likely to take the risk to leave a job even though they may not like it, even though they may not be performing overly well, even though they may not have their heart in it.
"They are unlikely now to take the risk of trying to get a new job because they could go to a new job and find within 90 days they are dismissed without any reason at all."
The extension of the 90-day rule was first proposed by the ACT Party and Mr Reid says it is a clear example of ACT wagging the dog's tail.
Changes beneficial, says McDonald's boss
The managing director McDonald's in New Zealand says the proposed changes are beneficial to employers and workers.
Mark Hawthorne says it is not in the company's best interest to fire people after 90 days.
"We do not want people leaving in the first 90 days. It costs us $2000 per person to train them and we also get globally measured on the number of people we lose within the 90 days."
However, Mr Hawthorne says the 90-day trial allows the company a certain flexibility.
"It's good for us to continue hiring people and to make sure that they are given a genuine go and we can do that knowing that, whilst we don't want to use that option, it is there."
Doctors believe they should be exempt
The Resident Doctors' Association also made submissions on Wednesday, telling the select committee it should be exempt from any law change.
Its president, Curtis Walker, says doctors are already thoroughly scrutinised before being hired and the provision will put them off working in New Zealand.
"We're already struggling to keep doctors in this country. To provide an additional layer of job insecurity, however unlikely it will be used on doctors, is just a further disincentive to keep working in New Zealand, and indeed for other doctors coming from overseas to work in New Zealand if they feel that they'll be placed under such a regime."
Dr Walker says that junior doctors could find themselves in a permanent state of limbo should the bill become law.
"With our training, we move district health boards regularly - sometimes as frequently as every two months to a different employer - so potentially, we are permanently in a state of new employment under this legislation, which places us potentially at the whim of a district health board."
The select committee is due to submit its report to Parliament in early November.
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