A Wellington contractor has been given six months' home detention after failing to pay nearly $240,000 in income tax and GST.

Regina Beck had been working as an independent contractor through her company, Ichange Management Consultants Limited. She was the sole director and shareholder.

Ichange didn't register for GST, despite earning more than the $60,000 annual income threshold, but charged GST on the contracting income it earned over a five-year period.

Income tax returns for the years 31 March 2007 to 31 March 2012 showed losses or a small profit, which was offset against losses carried forward. No tax had been paid by Ichange, although it received contracting income of about $700,000.

Inland Revenue Investigations and Advice Group Manager Patrick Goggin said Beck's offending was brazen and sustained, and showed total disregard for New Zealand's system of voluntary tax compliance.

"Most people in Beck's position do the right thing, declare all their income and pay their fair share of tax," Mr Goggin said.

"But for some reason she didn't play ball, and thought she could get away with not contributing any tax towards the social services we all benefit from."

When she was audited, Beck only disclosed work undertaken through a recruitment agency, failing to mention income earned from other sources by herself and Ichange.

An analysis of Ichange's bank statements identified further contracting income of $400,000 which had not been disclosed even after Ichange was put under audit.

It was established that $159,136 of income tax and $79,191 of GST had been evaded. Beck pleaded guilty to all charges.

New Zealanders are being warned about a spate of scam phone callers saying they are from Inland Revenue and trying to recover unpaid taxes.

Inland Revenue has received notification of the scam calls from more than 500 customers all over the country, with the first report on Tuesday morning.

Customer Services Group Manager Eleanor Young says the scammers generally allege the recipient of the call are being investigated for historic fraud or evasion, before threatening them with legal action.

In some cases people are told they must pay a debt urgently or face jail, and to avoid that fate are told to ring a Wellington number (04 830 2441) and speak to a Kenneth Matthews or a person with a similar sounding name.

"These callers are quite clever and are particularly aggressive, so people need to be wary of being caught out," Ms Young says.

"They identify themselves in the way an Inland Revenue representative would, and request personal information including the person's IRD number in a very convincing and confident manner."

"Some customers who are used to dealing with Inland Revenue have received these calls and say the scammers almost sound legit."

Calls have been made to both landline and mobile numbers, and messages have been left on customers' voicemails stating they are subject to criminal action for tax fraud.

"We recommend that if anyone receives a call of this nature they hang up and if they have any concerns to email phishing@ird.govt.nz."

Inland Revenue would never ask for bank account or PIN numbers over the phone in order to process a monetary payment.

"Customers should always be totally satisfied that they are talking to a genuine Inland Revenue staff member before handling over any personal details."