Are they in seed trays or individual pots? 5cm is getting a bit tall for tomatoes still to be in trays. You should prick them out into individual pots of seedling compost and when they are about 15 - 20cm tall and have two or more sets of true leaves you can plant them in their final positions. Can't help you with the melons really as I can't grow them in my climate but the same would apply to pricking out and then transplanting into final positions. BTW the rate of growth sounds normal to me but I do live in a cool climate. I sow tomatoes in late March to plant out in late May.

Welcome compostkid to the GardenWeb. When I read your post my first association of Victoria was not with Australia. I agree with flora's advice, but would suggest you edit your posting address to indicate your zone and your down under location. Al