For the purposes of insurance anything that should be road registered is considered to be a vehicle. That includes trailers. These must be insured on a vehicle insurance policy and won't be covered under a "mobile assets" or tools and equipment policy.

Some commercial vehicle policies automatically include cover for "unspecified trailers" up to a certain value. This means any light trailer that isn't explicitly listed on the policy can still be covered, up to that amount. In some policies this could be as low as $1,500. With Builtin it is $2,750.

So, if your $5,000 trailer that you forgot to insure gets stolen you may have some cover under this unspecified trailers benefit, up to the policy limit.

An even better idea is to add the trailer to your policy, so that it is insured for its full value. Generally, adding a trailer is inexpensive (around $220+GST at the time of writing) and they have a low excess ($100 at the time of writing).