In a word, yes.
For any Separation Agreement to be upheld and to help ensure a Separation Agreement is not challenged, you and your ex-partner must be fully open about your finances. This is called providing full and frank “financial disclosure”.
Without full and frank disclosure it will be difficult for any lawyer to advise you on whether any proposed agreement is fair and appropriate to the circumstances of your case. By having the benefit of financial disclosure, however, each of you will know what the other has by way of savings, investments, property and debts, and you can therefore agree what you’re each responsible for paying.
If you or your partner are not open and honest about your finances, it’s likely to mean the Separation Agreement cannot be relied upon or enforced.