Australian Olympic champion Libby Trickett has added another member to her bustling family by giving birth to her fifth child over the weekend.

The swimmer, 40, took to Instagram to share the heartwarming news with her 79,000 followers and announce that 'the shape' of her family is now complete.

'He's here,' the post began, alongside a photo of Trickett and husband Luke holding the newborn.

'Archie Wilson Lightning Trickett arrived at 7.51am on the 3rd of April via an elective caesarean birth, weighing 3.99kg and 53cm long.

'The shape of our family is complete.

'(Definitely promise it's for reals this time! I'm really *almost* certain this time.. though I've come to realise that I probably would have babies forever, I know that I've pushed my body to its absolute limits with this one).'

'Every birth I've had has been extraordinary and this one was no different. Calm, full of joy and laughter, I couldn't have asked for a better way to meet our baby boy,' she wrote.

'Archie is already getting suitably smothered by his 3 big sisters and big brother. We have all been struck with love.'

The four-time Olympic gold medallist also explained that she has made the same mistake while pregnant with all her kids.

'Just as a fun fact: we didn't find out the sex of the baby for any of our pregnancies, and I literally guessed the gender of every single one of my children, wrong. I didn't predict a single one and I feel so lucky because I couldn't have wished for more,' she wrote.