Is it safe for a baby to sleep in a portacot bassinet/cot as a transition from bassinet to full size cot? I have a 3 week old and he somehow wriggles to the edge of the bassinet where the sides are cotton not mesh and I’m worried about him sleeping.

It is ok for a newborn baby to go into a full size cot - ensure it meets Mandatory Australian Standards.
Are you wrapping/swaddling him? What are you using to wrap/swaddle?
Some very young babies do wriggle a lot when they don’t respond well to swaddling/wrapping.  Some like to touch their face &/or suck on their fingers to settle.