Millions of dollar are going back into the Government's back pockets in unclaimed and uncollected tenancy bonds.

Renters are unwittingly gifting the Government millions of dollars in tenancy bonds.

Documents obtained under the Official Information Act show for the five years to 2014, more than $17 million in unclaimed or uncollected tenancy bonds has been reclaimed by the Ministry of Business, Innovation and Employment (MBIE).

An unclaimed bond refers to a tenancy bond that has not been claimed within two months of the tenancy being terminated. An uncollected bond means an application to retrieve the money has been processed and the payment approved but the money was not collected within two months.

Under the Residential Tenancies Act 1986, after six years that money then belongs to the Crown.

In that account, there is $44.6m of unclaimed and uncollected bonds relating to 142,864 tenancies. Already, for the first three months of 2015, there is already almost $640,000 in unclaimed bonds and $1.3m uncollected.

Christchurch Tenants' Protection Association educator and advocate Lisa Coulter said the money was often left dwindling in the Government's accounts because tenants were generally unaware of their rights.

"It's a huge amount of money – it's really disappointing," she said.

"What tends to happen is the landlord tells the tenant what's going to happen with their bond money instead of asking them."

Tenants often believed the decision on what happened to the bond was solely up to the landlord, Coulter said, but did not realise their signature was also required before any money was released.

Common reasons behind bonds not being claimed were tenants simply wanting to move on from their past tenancy quickly, trouble with bank processing, disputes with landlords that can drag on, or delays when one person in a tenancy changed and the bond centre had to spend time updating files, she said.

One of the most common mistakes was people having their tenancy address as the "address for service" on their tenancy form and moving when a tenancy ends, making it difficult to track the person down.

"Decisions around bonds and how they're going to be refunded seem to be taking an awful lot longer than they used to be. People often call us frustrated at the timeline," Coulter said.

"The landlord says 'oh well, that's going to be taken out of their bond' and there's an assumption it's all gone."

Coulter said there were ways tenants could protect themselves, most importantly having an alternative address as their address for service and knowing their rights as renters.

"Be aware the bond money is theirs and it's reasonable to expect that the process is done as quickly as possible, as there is a real impact."

MBIE spokesman Mike West said the ministry was "committed to ensuring that tenants and landlords have access to their bonds".

"The ministry also takes all reasonable steps to locate and contact tenants and landlords after their tenancy has ended," he said.