Is it safe for my 7 month old to sleep in a portacot for 4-5 weeks? I have read elsewhere, that they are only suitable for a couple of nights and not longer term. Thanks

Product Safety Australia recommends that a portable cot should be used as a temporary sleep space. this is due to the fact that they are not as sturdy as a standard household cot & are more likely to develop “wear & tear”.
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/folding-cots

It is important to check all instructions that come with the portacot, as some portacots recommend using once baby reached a specific weight - usually 15kg but some recommend only till 8kg.

Red Nose has the following information:
Only use the firm, thin, well-fitting mattress that is supplied with the portable cot. Never add a second mattress or additional padding under or over the mattress, which has been specifically designed for the portacot, as baby may become trapped face down in gaps between the mattress and the sides.

Wear and tear can expose baby to hazards. Only use a portable cot that has the mesh intact and that has no broken parts. Ensure that the base is flat and regularly check the portacot for signs of damage.
https://rednose.org.au/article/portable-cots

It is difficult to state exactly what is short term use & what is long term use.
However, it is important to consider the weight of the baby, correct assembly of the cot & to check regularly for wear & tear when using a portacot.