It would be good if anyone with information specifically relating to the legality of owning handcuffs in various jurisdictions could add to this thread and include information about weather or not restraints are permissible in their states/countries. Hopefully that way we can create a database of knowledge around this specifically.
After looking around at the legality of owning handcuffs in Australia, the answer is... It depends. There doesn't appear to be any national law, however for the East Coast the provisions in Australian law don't seem to cater for the ownership of handcuffs for recreational purposes really.
For Australia (The state of Queensland specifically) http://www.legislation.qld.gov.au/legisltn/current/w/weaponscatr97.pdf
9 Restricted items (Act, s 67)
The following items are restricted items for section 673 of the
Act—
(a) handcuffs, thumbcuffs or other similar restraints;
snip
67 Possessing and acquiring restricted items
(1) A person must not, without reasonable excuse, possess or
acquire a restricted item.
Maximum penalty—10 penalty units.
(2) For subsection (1), it is not a reasonable excuse to possess or
acquire a restricted item for crowd or traffic control.
(3) In this section—
restricted item means an item prescribed as a restricted item
under the Weapons Categories Regulation 1997.
For Australia (The state of Victoria) http://www.police.vic.gov.au/content.asp?Document_ID=28555
A person must be able to demonstrate both of the following two requirements in order to lawfully use handcuffs when working in the private security industry.
Requirement 1: Training
The individual licence or permit holder must have completed the unit of competency 'Restrain Persons Using Handcuffs' (CPPSEC3015A).
Requirement 2: Permission from your employer
The licence or permit holder must receive approval from their employer before they can use handcuffs as part of their job.
Employees are responsible for providing their employer with evidence that they have completed the relevant training. Employers or business licence holders are responsible for determining whether it is appropriate for their staff to use handcuffs at work. Employers are also responsible for checking that their employees have completed the relevant training at an approved security training organisation.
A member of Victoria Police may request persons working with handcuffs to demonstrate that the above requirements have been met.
For Australia (The State of New South Wales NSW) http://www.police.nsw.gov.au/__data/assets/pdf_file/0020/185141/Batons_and_Handcuffs_FACT_SHEET_2013_1.3.pdf and http://www.legislation.nsw.gov.au/fullhtml/inforce/act+127+1998+FIRST+0+N?
Considered Schedule 1 Prohibited weapons
. There are however the risk of having and using them to hold someone on scene that you as the "operator" of said cuff can be held liable for any and all damages to the person being cuffed if it can be substantiated that his/her injuries were or are a result of you incorrectly using or applying the cuffs.