I have a 14 week old and a 20 month old. We have just purchased a brand new caravan with double bunks. I know bub is too little at the moment but when would it be safe for my 14 week old to move from bassinet to bunk? The bunk has a wall on 3 sides and bunk safety net on the open side.

Once baby is showing attempts to roll, then they need to be moved from a bassinet to a cot & not be swaddled.
Babies need arms free & the space of a cot to roll well.
https://rednose.org.au/article/bassinets

Recommendation for when to move a child from a cot to bed is once they are attempting to climb from cot - this usually occurs from about 2.5 years or can be as early as 18mths.
The cot to bed brochure (linked below) has some helpful tips about safety for child once they are no longer in a cot.
https://rednose.org.au/downloads/RN0407_22_Cot-to-Bed.pdf

There are many things to look for when using a bunk bed for children, particularly the top bunk.
Most safety organizations recommend that a child is not placed on top bunk until they are 9 years of age.
The most common injuries to very young children using bunk beds are falls & risk of entrapment.
https://rednose.org.au/article/what-is-a-safe-bunk-bed
https://rednose.org.au/article/when-can-i-sleep-my-child-in-a-bunk-bed

While portable bunk beds or bunk beds for caravans/trailers are not included in Product Safety Australia (ACCC) Mandatory Standards for household bunk beds, there are some ideas that you might find helpful in assessing the safety of the bunk beds in your caravan.
https://www.productsafety.gov.au/business/search-mandatory-standards/bunk-beds-mandatory-standard