Hi @zodiac1818,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The standard maximum joist spacing for almost all decking, including 140 x 19mm Merbau is 450mm.
Further details can be found in the product brochure under the heading installation guidelines.
You might like to check out the following articles for some help designing your deck -
Let me know if you have any further questions.
Jacob
