Employers were reminded that part-time employees are entitled to pro-rated holidays and leave.

The University of Canterbury Students Association has been asked to repay employees after it breached their holiday pay entitlements.

The students association (UCSA) was issued with an improvement notice after receiving a complaint from a former employee who claimed he had not received the correct entitlements for annual leave and sick leave.

A Labour Inspectorate investigation found at least 14 UCSA employees were not receiving their holiday and sick leave entitlements because they were described by the association as "casual" but were found to be part-timers.

The inspectorate is part of the Ministry of Business, Innovation and Employment (MBIE), which on Wednesday reminded employers that part-time employees who work regular rostered hours are entitled to pro-rated holidays and leave.

The inspectorate regularly saw cases where people working part-time were not getting their legal entitlements because the employer assumed they could just pay 8 per cent on top of their usual pay to cover off their holiday entitlements, Labour Inspectorate southern regional manager Stuart Lumsden said.

"Employers need to remember that it is only acceptable and lawful to add eight per cent of the employee's gross earnings as annual holiday pay to their regular pay packets for employees who are on a fixed term contract or who work irregularly or intermittently – for instance on-call workers," Lumsden said.

"In all other cases the four weeks' pro-rated rule applies."

Staff who worked a regular pattern, even one hour a week, were part-timers while those with irregular patterns determined by the employer were casual.

Lumsden said there were dozens of such cases each year across a range of employment sectors. The inspectorate would take action if encountered these breaches of employment law.

The inspectorate issued an improvement notice to the UCSA after receiving a complaint from a former employee who claimed he had not received the correct entitlements for annual leave and sick leave. An investigation found 14 employees in a similar position.

"In reality, they were part-time employees whose work patterns were established on a roster. They were therefore entitled to four weeks' holiday pay, pro-rated, under the Holidays Act 2003," he said.

"Employees who have been working with the employer for six months or more (even part-time) and casual employees were also entitled to five days' sick leave, if they meet a certain criteria."

Lumsden said UCSA repaid the 14 employees more than $15,000 in arrears and rectified its practices. The breaches dated back a year or so.

Employers needed to ensure they comply with employment standards as failure do so not only harmed their staff but was ultimately a cost to their business.

The onus was on the employers to check with MBIE on what entitlements their staff were entitled to.

UCSA chief executive Dave Hawkey and student president Sarah Platt both said the association was not commenting on employment matters in this instance.