plumbing piece is too big for the plumbing on the wall
Hello @sarahd94
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your plumbing question.
Your tap currently has a 15mm outlet and your plastic fitting uses 20mm. I propose using a Brasshards 20 x 15mm Brass Threaded Hex Reducing Bush LF in order for your fitting to work. Please make sure to apply FIX-A-TAP 12mm x 8m Blue PTFE Thread Seal Tape on the wall tap before screwing the brass adapter onto it.
If you need further assistance, please let us know.
Eric
