The number of people deported from Australia because of serious criminal convictions has increased more than tenfold since 2014, with a huge spike in the number of New Zealanders being sent home - one of two scathing ombudsman's reports on Monday on practices by Australian immigration authorities.

The review of section 501 of the Migration Act considered the treatment of people who have their Australian visas cancelled because of crimes resulting in more than 12 months' imprisonment, finding the number of visas cancelled shot up from 76 in 2013-14 to 983 in the last financial year.

Of the 1219 non-citizens who had their visas cancelled between January 2014 and February 2016, 697 were from New Zealand and 124 were from Britain, many of whom have been in Australia since childhood.

The Ombudsman found the federal government had failed in its promise to cancel visas well before an individual's expected release date from prison, causing prolonged family separation and undermining consideration of the best interests of children.

The report found serious delays in deciding the outcome of revocation requests meant former prisoners were spending long periods in limbo inside immigration detention.

"The delays in deciding revocation requests undermines the department's policy of giving primary consideration to the best interests of those who have young children and/or experience prolonged family separation," Commonwealth Ombudsman Colin Neave said.

The deportations have caused disquiet at the highest levels of the trans-Tasman relationship. In October 2015, former New Zealand prime minister John Key raised the issue with Malcolm Turnbull, citing the cases of hundreds of Kiwis facing deportation and being held for months in detention while their claims were processed.

The Ombudsman's report recommended better co-operation between state and federal authorities, prioritisation for cases of people with responsibilities to care for children and long-term residents, and new standard timeframes for processing cases.

A spokeswoman for Immigration Minister Peter Dutton said the government made no apologies for strengthening deportation laws "to further protect the Australian community".

"Since the laws were strengthened, the government has cancelled the visas of serious criminals including 158 visas for child sex offences, 361 for assault, 78 for rape and 33 for murder, to name a few."

The second report found many people who had bridging visas cancelled by the government because of a criminal charge, conviction, or threat posed to the community faced "prolonged and potentially indefinite periods of immigration detention".

Investigators received complaints about individuals being held in immigration detention for prolonged periods after criminal charges were withdrawn or where the person was acquitted, fined or given a good behaviour bond, and said some people on bridging visas were being denied the presumption of innocence.

"To suddenly deny a person their liberty to live freely in the community based on nothing more than an allegation that has led to the laying of criminal charges, raises the question of whether the department is acting prematurely by cancelling a visa and whether the department is not following the spirit of the explanatory statement that introduced this legislation," the report said.

The department was unable or unwilling to provide the investigation with key data and information, in some instances despite repeated requests.

The report recommended adequate time and resources be given to people having their visas cancelled to seek legal advice and make a case against the cancellation, that proper explanation of cancellations be provided in the individual's own language, that people not be transferred between detention centres until the appeal period expires and for the minister to act promptly in some cases.

"The ongoing detention of many individuals in this cohort is inappropriate and has negatively impacted upon their mental health," Mr Neave said.

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